Initial commit: Sizzletracker Android music tracker

A retro, grid-based music tracker for Android (Kotlin + Jetpack Compose,
single Activity) with four equally-capable input methods (touch, keyboard,
gamepad, MIDI) and four tabs: tracker, mixer, toolbox, settings.

Highlights:
- Tracker: Canvas-drawn 4-track pattern grid over an 8-lane arrangement roll,
  with a glyph cache and draw-phase state reads so the playhead and edits
  redraw without per-frame recomposition.
- Audio: sample-accurate sequencer feeding a shared AudioEngine, driven by
  either a Kotlin AudioTrack loop or native Oboe/AAudio via JNI (16 KB-aligned
  native libs). media3 MediaSession for lock-screen/headset transport.
- Toolbox: 16 instrument/effect slots with a 2-octave audition keyboard;
  single-tap select, double-tap edit, long-press clear.
- Note entry: long-press cell popups (piano keyboard / value steppers) plus
  keyboard/gamepad stepping that resumes from the last note/channel entered.
  Velocity capped at 0x7F, channel at 16.
- Selection/clipboard (cut/copy/paste/delete) and .sng import/export
  compatible with the reference desktop tool.
- A `profile` build type (non-debuggable, debug-signed) for realistic
  on-device performance testing.
- Developer handover documentation under docs/.

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

samples/demo.sng

@@ -0,0 +1,28 @@
+SIZZLETRACKER-SNG 1
+NAME=demo
+TEMPO=120.0
+SIG=FOUR_FOUR
+SCALE=MINOR
+ROOT=9
+ACTIVE=0
+[PATTERN id=0 name=LEAD len=16]
+0,0,57,128,1
+0,4,60,128,1
+0,8,64,110,1
+0,12,60,120,1
+1,0,45,90,2
+1,8,45,90,2
+[ARRANGEMENT len=16 loop=0]
+0,0,0
+0,1,0
+0,2,0
+0,3,0
+[REGION A en=0 s=0 e=0 r=2]
+[REGION B en=0 s=0 e=0 r=2]
+[MIXER]
+0,0,-1,-1,-1,-1,1,0.8,0,0
+1,-1,-1,-1,-1,-1,2,0.8,0,0
+2,-1,-1,-1,-1,-1,3,0.8,0,0
+3,-1,-1,-1,-1,-1,4,0.8,0,0
+[TOOLBOX]
+0=NES_SYNTH;Lead;wave=Pulse25,attack=0.01,decay=0.15,sustain=0.6,release=0.1