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),