Add detailed user guide with screenshots

docs/USER_GUIDE.md: a full walkthrough of every screen — Tracker (transport,
grid, note-offs, scrolling), the punch-in keyboard, Arrangement, Mix
(channels, FX routing, recording), Toolbox (tile gestures, Synth/Sampler/
SF2-XI editors, effects, presets) and Setup (projects, audio routing, themes,
input bindings). Also covers the four input methods incl. the new
hold-to-repeat, file formats, recording, and default keyboard/gamepad/MIDI
maps. 11 on-device screenshots under docs/screenshots/, linked from README.

Also refresh the README's stale "NES synth" wording to "3-oscillator synth"
(renamed in v0.5.0).

Docs only — no code change, so no version bump or rebuild.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -11,6 +11,9 @@ gamepad, and MIDI** (USB or Bluetooth).
   <img alt="license" src="https://img.shields.io/badge/license-GPL--3.0--or--later-e05a4b">
 </p>
 
+📖 **[Full User Guide](docs/USER_GUIDE.md)** — a detailed, screenshot‑by‑screenshot
+walkthrough of every tab, instrument, effect and control.
+
 ## Features at a glance
 
 - **Tracker tab** — 4-track pattern grid (note / velocity-hex / MIDI-channel per
@@ -19,7 +22,7 @@ gamepad, and MIDI** (USB or Bluetooth).
   with A/B loop regions.
 - **Mix tab** — 4 channels, each with an instrument + 4 FX slots, MIDI channel,
   volume, mute/solo.
-- **Toolbox tab** — 16 slots for instruments (NES synth, sampler, SF2/XI) and
+- **Toolbox tab** — 16 slots for instruments (3-oscillator synth, sampler, SF2/XI) and
   effects (arp, transposer, LFO, tape delay, reverb, filters, 10-band EQ,
   bitcrusher). Every device has an auto-generated editor and text presets.
 - **Setup tab** — `.sng` project save/load, color themes (with import/export),

docs/USER_GUIDE.md

@@ -0,0 +1,417 @@
+<!-- SPDX-FileCopyrightText: 2026 Reactorcoremeltdown -->
+<!-- SPDX-License-Identifier: GPL-3.0-or-later -->
+
+# Sizzletracker — User Guide
+
+Sizzletracker is a retro, demoscene‑flavoured **music tracker for Android**. You
+sequence notes on a fixed‑width grid, build a song out of reusable pattern
+**blocks**, and play everything through built‑in synths, samplers and effects — a
+low‑latency native audio engine driving it all. Everything can be driven equally
+by **touch, a physical keyboard, a gamepad, or MIDI**.
+
+> **Screenshots** in this guide were taken on a phone in the *Vintage Beige*
+> theme. Your colours will differ if you pick another palette, and any floating
+> buttons you see over the app in a shot belong to a different app, not
+> Sizzletracker.
+
+---
+
+## Contents
+
+1. [Requirements & install](#requirements--install)
+2. [Core concepts](#core-concepts)
+3. [Getting around: the four tabs](#getting-around-the-four-tabs)
+4. [Tracker tab](#tracker-tab)
+5. [The punch‑in keyboard](#the-punch-in-keyboard)
+6. [Arrangement](#arrangement)
+7. [Mix tab](#mix-tab)
+8. [Toolbox tab](#toolbox-tab)
+   - [Instruments](#instruments)
+   - [Effects](#effects)
+   - [Presets](#presets)
+9. [Setup tab](#setup-tab)
+10. [Input devices & hold‑to‑repeat](#input-devices--hold-to-repeat)
+11. [Files, projects & sharing](#files-projects--sharing)
+12. [Recording](#recording)
+13. [Default control maps](#default-control-maps)
+14. [Tips & troubleshooting](#tips--troubleshooting)
+
+---
+
+## Requirements & install
+
+- **Android 8.0 (Oreo) or newer.** The app relies on the native MIDI API and
+  low‑latency AAudio, which need this floor.
+- Audio uses **Oboe** (low‑latency) with an automatic fallback to the standard
+  path if Oboe can't start on your device.
+- Installed as an APK (e.g. from a personal F‑Droid repository). Package id:
+  `space.rcmd.android.sizzle`.
+
+---
+
+## Core concepts
+
+A few ideas make the whole app click:
+
+- **Tracks vs. channels.** The pattern grid has four vertical **tracks** (columns
+  of notes). Each note carries a **MIDI channel** number; that channel decides
+  which **mixer channel (bus)** actually sounds it. So *where* a note sits in the
+  grid is independent of *which instrument* plays it — routing is by channel.
+- **Blocks.** A **block** is a short pattern. You compose blocks in the Tracker,
+  then lay them out over time in the **Arrangement** to form the full song.
+- **The Toolbox** holds up to 16 **devices** — instruments (Synth, Sampler,
+  SF2/XI) and effects (delay, filter, reverb, MIDI arpeggiator, …). The Mix tab
+  assigns one instrument and up to four effects to each of the four channels.
+- **Four equal inputs.** Touch, keyboard, gamepad and MIDI are all translated
+  into the same internal actions, so every feature works from every device.
+
+---
+
+## Getting around: the four tabs
+
+The bottom bar switches between the four main screens:
+
+| Tab | What it's for |
+|-----|---------------|
+| **Tracker** | Enter notes, edit patterns, arrange blocks into a song |
+| **Mix** | Assign instruments/effects to channels, set levels, arm recording |
+| **Toolbox** | Create and edit the 16 instrument/effect devices, manage presets |
+| **Setup** | Projects, audio routing, themes, input bindings, MIDI panic |
+
+---
+
+## Tracker tab
+
+<img src="screenshots/01-tracker.png" width="300" alt="Tracker tab">
+
+The top of the screen is the transport and editing toolbar; below it is the
+pattern grid; below that (in portrait) is the Arrangement.
+
+**Transport row**
+
+- **⧉ STOP** — stop playback. Pressing STOP again when already stopped rewinds to
+  the start.
+- **▶ PLAY / ❚❚ PAUSE** — start or pause. When the arrangement is empty, the
+  currently‑edited pattern loops so the Tracker is always audible while you work.
+- **– / + BPM** — nudge the tempo. Hold to repeat.
+- **SIG** — time signature (beats per bar / beat unit).
+
+**Pattern row**
+
+- **BLK** — which block you're editing.
+- **LEN** — the block's length in lines.
+- **SCALE** — the musical scale used when entering/transposing notes (e.g.
+  *Chromatic*). Note edits snap to this scale.
+- **ROOT** — the scale's root note.
+
+**Editing row**
+
+- **SEL** — start/extend a selection.
+- **✂ / ⧉ / 📋** — cut, copy, paste the selection.
+- **✕** — clear the focused cell(s).
+- **OFF ===** — place a **note‑off** on the focused track (see below).
+- **SKIP** — how many lines the cursor jumps after each note entry (step size).
+- **KBD** — show/hide the punch‑in keyboard.
+
+**The grid**
+
+Each row is one line (numbered in hex on the left). Each of the four tracks shows
+three columns:
+
+- **Note** — e.g. `C‑4`, or `···` when empty, or `===` for a note‑off.
+- **Velocity** — how hard the note plays (hex), or `..` when empty.
+- **Channel** — the MIDI channel that routes the note to a mixer bus.
+
+The **cursor** highlights the focused cell; tap a cell to move there. The
+**playhead** row is highlighted while playing. Beat and bar lines are tinted so
+you can read the meter at a glance.
+
+**Scrolling & follow.** During playback the grid auto‑centres on the playhead;
+flick up/down to detach into free **inertial scrolling**; switching to another
+block re‑attaches the auto‑follow.
+
+**Note‑offs are tied to their track.** A `===` cell has no note, velocity or
+channel of its own — it simply releases whatever that track last started. This is
+how you end a sustained note cleanly.
+
+---
+
+## The punch‑in keyboard
+
+Tap **KBD** to reveal a two‑octave on‑screen piano fixed at the bottom.
+
+<img src="screenshots/02-tracker-keyboard.png" width="300" alt="Punch-in keyboard">
+
+The bar above the keys sets what you're punching in:
+
+- **CH** — the MIDI channel written into new notes (routes them to a mixer bus).
+- **OCT** — the octave pair shown on the two rows.
+- **VEL** — the velocity written into new notes.
+
+Playing a key enters a note at the cursor and advances by the **SKIP** amount.
+The same keyboard is also used purely to *audition* instruments in the Toolbox.
+
+---
+
+## Arrangement
+
+Below the grid is the Arrangement — a timeline of your blocks.
+
+- Rows are **lanes** (independent parts that play together); columns are beats.
+- Paint a block into a lane so it sounds at that point; a **contiguous run** of the
+  same block plays it through beat‑by‑beat.
+- **M** mutes a lane.
+- **LOOP** loops the arrangement; **A** / **B** define two loop regions, each with
+  its own repeat count (the **–/×N/+** controls).
+- **FILL** helps lay a block across a span quickly.
+
+The canvas extends up to 256 bars, so full songs fit.
+
+---
+
+## Mix tab
+
+<img src="screenshots/03-mix.png" width="300" alt="Mix tab">
+
+Four channels, side by side. For each channel:
+
+- **INSTRUMENT** — the Toolbox device that makes its sound.
+- **FX 1–4** — up to four insert effects, applied in order.
+- **MIDI CH** — which MIDI channel this bus listens on. Notes whose channel
+  matches play here. (Set two buses to the same channel to layer them.)
+- **VOLUME** — drag the fader.
+- **LIM** — a per‑channel soft‑clip limiter.
+- **M / S** — mute / solo.
+
+The strip along the top controls **recording** of the master output:
+
+- **● ARM** — arm recording; capture begins when you press play.
+- **TAIL** — how many bars of effect tail to keep after the sequencer stops.
+- **DIR** — the destination folder for the rendered file.
+- **master → WAV** — the master bus is captured to a stereo WAV.
+
+---
+
+## Toolbox tab
+
+<img src="screenshots/04-toolbox.png" width="300" alt="Toolbox grid">
+
+A 4×4 grid of 16 device slots. Filled tiles show the device name and whether it's
+an instrument (**INS**) or effect (**EFF**).
+
+**Tile gestures**
+
+- **Single tap an empty tile** → choose a device to load.
+
+  <img src="screenshots/05-device-picker.png" width="300" alt="Device picker">
+
+- **Single tap a filled tile** → open its editor.
+- **Long‑press a filled tile** → clear it (a confirmation appears first, since
+  clearing discards the device and its settings and can't be undone).
+
+The keyboard pinned at the bottom of the Toolbox auditions the current device.
+
+### Instruments
+
+**Synth** — a three‑oscillator subtractive synth (Minimoog‑style): three
+oscillators → mixer → resonant low‑pass filter (with its own envelope and key
+tracking) → amplifier, plus a tempo‑synced LFO and a small modulation matrix. The
+editor is paged (swipe, or tap the dots): **Oscillators**, **Filter**, **Amp +
+LFO**, **Mod**.
+
+<img src="screenshots/06-synth-editor.png" width="300" alt="Synth editor — oscillators">
+
+Controls are **rotary knobs**: slide a finger up/down to change a value (a full
+sweep spans a comfortable travel distance, so fine tweaks are easy), and
+**double‑tap a knob to reset** it to its default. Factory presets include
+chiptune tones and a darkwave/industrial set (plucky basses, cutting leads, and
+harsh detuned deep basses for layering).
+
+**Sampler** — a 16‑pad drum/sample bank. Each pad maps to a consecutive note from
+C2 upward.
+
+<img src="screenshots/08-sampler-editor.png" width="300" alt="Sampler editor">
+
+Per pad you can **IMPORT WAV**, **RECORD**, **CLIP** (trim with the waveform
+markers), **NORMALIZE**, or **CLEAR**, and set a per‑pad **volume**. Tap a pad in
+the grid to play and select it.
+
+**SF2 / XI Loader** — loads a single sample/instrument from a SoundFont (`.sf2`)
+or FastTracker (`.xi`) file, pitched across the keyboard from its root note.
+
+<img src="screenshots/07-soundfont-editor.png" width="300" alt="SF2/XI loader editor">
+
+It has a volume **ADSR envelope** and an optional **LOOP** (sustains the sample
+until note‑off) with an adjustable crossfade (**XFADE**) to smooth the loop seam.
+
+### Effects
+
+Insert effects (assign them to a channel's FX slots in the Mix tab):
+
+| Effect | What it does |
+|--------|--------------|
+| **Tape Delay** | Up to four tempo‑synced tape "heads", with feedback, dry/wet and tape character (saturation, darkening, wow/flutter) |
+| **Filter (LP/HP/BP)** | Resonant low/high/band‑pass filter with warmth and a signal‑triggered cutoff envelope |
+| **10‑Band EQ** | Ten‑band graphic equaliser |
+| **Bitcrusher** | Bit‑depth and sample‑rate reduction with drive |
+| **Ambience Reverb** | A multi‑algorithm reverb with many factory spaces |
+| **MIDI Arpeggiator** | Turns held notes into tempo‑synced octave patterns |
+| **MIDI Transposer** | Shifts incoming notes by octaves/semitones |
+| **MIDI LFO** | Modulates a chosen parameter of another device |
+
+The Tape Delay editor, as an example — drag a head's fader to set its time, tap
+its number to switch it off/on:
+
+<img src="screenshots/09-tape-delay-editor.png" width="300" alt="Tape Delay editor">
+
+> **MIDI effects vs. audio effects.** The Arpeggiator, Transposer and LFO shape
+> notes/parameters before the sound is made; the rest process audio. All are
+> managed the same way — as devices you drop into a channel's FX slots.
+
+### Presets
+
+Every device editor has a preset bar at the top:
+
+- **PRESET** dropdown — load a saved preset.
+- **SAVE** — save the current settings as a named preset.
+- **DELETE** — remove the selected preset (asks to confirm).
+- **IMPORT** — load a preset from a file (Sampler presets can be imported from a
+  `.zip` bundle that carries the sample).
+- **SHARE** — send the preset via the system share sheet.
+
+Presets are plain text (`.szp`), stored per device type in the app's storage.
+Factory presets are seeded automatically and reappear as new ones ship, but ones
+you delete stay deleted.
+
+---
+
+## Setup tab
+
+<img src="screenshots/10-setup.png" width="300" alt="Setup tab">
+
+- **MIDI PANIC — ALL NOTES OFF** — instantly silences every voice. Use it if a
+  note ever hangs.
+- **Project** — save / new / delete full projects on the device, and **Export /
+  Import `.sng`** for desktop interchange. (See [Files](#files-projects--sharing).)
+- **Audio Devices** — route **Output** and **Input** to USB or built‑in.
+- **Color Theme** — pick a palette, or **Export / Import `.szt`** themes. Stock
+  themes can't be deleted; imported ones join the list.
+
+Scrolling down reveals the input‑binding editors:
+
+<img src="screenshots/11-setup-bindings.png" width="300" alt="Setup — input bindings">
+
+- **Gamepad Bindings** — rebind buttons and button‑combos (chords).
+- **Keyboard Hotkeys** — rebind physical‑keyboard keys.
+- **MIDI Bindings** — MIDI‑learn: map any knob/pad to an action.
+
+---
+
+## Input devices & hold‑to‑repeat
+
+Sizzletracker treats **touch, keyboard, gamepad and MIDI** as equals — every
+action is available from each.
+
+- **Touch** — tap the grid, drag faders/knobs, play the on‑screen keyboard.
+- **Physical keyboard** — a built‑in "tracker piano" for note entry (see the map
+  below) plus rebindable hotkeys for navigation, transport and editing.
+- **Gamepad** — face buttons, shoulders, D‑pad and both analog sticks are all
+  bindable and can be combined into chords (hold one control as a modifier).
+- **MIDI** — notes play live; Control‑Change messages drive bound actions, and
+  unmapped ones can be MIDI‑learned in Setup.
+
+**Hold‑to‑repeat.** Holding a **navigation** control now auto‑repeats it — cursor
+Up/Down/Left/Right, column moves, and value increment/decrement. It fires once,
+then accelerates while held, and stops on release. This works identically on the
+keyboard, the gamepad (buttons, D‑pad **and** analog sticks) and a momentary MIDI
+control — so scrolling through a long pattern or ramping a value is a single hold.
+One‑shot actions (play, stop, tab switch, note/clear) still fire once even if held.
+
+---
+
+## Files, projects & sharing
+
+| Format | Contains | Use it for |
+|--------|----------|-----------|
+| **Full project** | Notes, blocks, arrangement **+ mixer, FX, instruments and sample assignments** | Your working song on this device |
+| **`.sng`** | Notes, blocks, arrangement only | Desktop‑compatible interchange |
+| **`.szp`** | One device's settings (Sampler as a `.zip` with its sample) | Sharing instrument/effect presets |
+| **`.szt`** | A colour palette | Sharing themes |
+
+Projects auto‑save, so a crash or process death resumes where you left off.
+Export a `.sng` before wiping data if you want a portable copy of the song.
+
+---
+
+## Recording
+
+1. In the **Mix** tab, set **DIR** (destination) and **TAIL** (bars of effect
+   tail to keep), then press **● ARM**.
+2. Press **PLAY**. Capture starts with playback and records the **master** bus.
+3. Recording ends automatically the set number of tail bars after the sequencer
+   stops. The finished stereo **WAV** is written to your chosen folder.
+
+---
+
+## Default control maps
+
+### Keyboard
+
+**Hotkeys** (rebindable in Setup → Keyboard Hotkeys):
+
+| Key | Action |
+|-----|--------|
+| Arrow keys | Move cursor (hold to repeat) |
+| Tab / Shift+Tab | Next / previous column |
+| Space | Play / pause |
+| Enter | Stop |
+| L | Toggle loop |
+| Page Up / Page Down | Increment / decrement value (hold to repeat) |
+| Delete | Clear cell |
+| `` ` `` (grave) | Note‑off |
+| – / = | Octave down / up |
+
+**Tracker piano** (fixed, for note entry):
+
+- Lower octave: `Z S X D C V G B H N J M`
+- Upper octave: `Q 2 W 3 E R 5 T 6 Y 7 U`
+
+### Gamepad (default layout)
+
+| Control | Action |
+|---------|--------|
+| A / B | Play‑pause / Stop |
+| X / Y | Clear / Toggle loop |
+| L1 / R1 | Previous / next tab |
+| D‑pad / Left stick | Navigate (hold to repeat) |
+| Right stick ↑ / ↓ | Increment / decrement (hold to repeat) |
+
+Bind a control *while holding another* to create a chord (e.g. "A + Left").
+
+### MIDI
+
+Note‑on/off play live. Bind Control‑Change numbers to actions in **Setup → MIDI
+Bindings** (or MIDI‑learn them). Momentary nav CCs auto‑repeat while held.
+
+---
+
+## Tips & troubleshooting
+
+- **A note is stuck.** Setup → **MIDI PANIC** silences everything immediately.
+- **No sound from a track.** Check the note's **channel** matches a mixer
+  channel's **MIDI CH**, that the channel has an **instrument**, and that it isn't
+  **muted** (or another channel **solo**ed).
+- **Crackles/dropouts.** The engine adapts its buffer automatically; make sure no
+  other heavy app is competing, and try routing output to a different device in
+  Setup → Audio Devices.
+- **A preset/instrument you want to keep.** Clearing a tile can't be undone —
+  **SAVE** it as a preset first. Long‑press asks before clearing for this reason.
+- **Moving a song to another app.** Export a **`.sng`** (notes/blocks/arrangement
+  are desktop‑compatible); keep a **full project** for the device‑specific mix and
+  samples.
+- **Auditioning fast.** Open any instrument in the Toolbox and use the pinned
+  keyboard; hold nav controls to move around quickly thanks to hold‑to‑repeat.
+
+---
+
+*Sizzletracker is free software, licensed GPL‑3.0‑or‑later.*