Glissando: slide across stacked keyboards to play (v0.7.3)

Input moves from per-key gestures to one octave-level multi-touch handler in
PianoOctave, so a finger can slide across keys and each note retriggers as it
crosses a key boundary (black keys win in their upper region via geometry-aware
hit-testing); multiple fingers still play independently for chords. Out-of-scale
keys are skipped mid-slide. onNoteOn carries a `slid` flag so the tracker
punch-in keyboard records only the initial press (a slide auditions a glide
without spraying a run of notes into the pattern), while the toolbox / editor
audition keyboards play every key crossed. StackedKeyboard and SlotAuditionKeyboard
both migrate to the new callback API; PianoOctave now renders the keys itself.

Verified on-device: a slow slide lights each in-scale key as the finger reaches
it and skips out-of-scale keys.

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 = 21
-        versionName = "0.7.2"
+        versionCode = 22
+        versionName = "0.7.3"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"