UI smoothness: distinct isPlaying flow + allocation-free grid draw (v0.5.1)

Two changes that cut playback-time UI jank (measured on-device):
- Expose a deduped isPlaying StateFlow (maps the transport flow through
  distinctUntilChanged) and collect THAT in the transport toolbar, so the
  PLAY/PAUSE button recomposes on play↔stop instead of ~12×/second as the
  playhead advances (the full transport emits every line).
- Precompute the pattern grid's fixed-width text (gutter hex, note names,
  velocity/channel) into class-load lookup tables. The grid redraws on
  every playhead line and scroll pixel, and the old toString/padStart
  formatting allocated a String per cell per frame — steady draw-phase
  garbage. The draw is now allocation-free (also improves GlyphCache hit
  rate via stable string instances).

gfxinfo over ~12s of playback on the busy track (moto g84):
  janky frames 47.2% → 32.0%; 99th-pct frame 129ms → 53ms;
  missed vsync 40 → 10; 50th 18ms → 14ms.

No engine/audio change. Remaining jank is the auto-follow scroll
redrawing the whole grid each line (everything shifts), not addressed here.

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

app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         // (android.media.midi) and AAudio low-latency audio we rely on.
         minSdk = 26
         targetSdk = 34
-        versionCode = 15
-        versionName = "0.5.0"
+        versionCode = 16
+        versionName = "0.5.1"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"