Fix stuck audition notes under rapid / chord tapping (v0.8.1)

The synth/SF2 audition note-off matched a voice purely by pitch, and isActive
stays true through a voice's release tail. So tapping a key, then immediately
holding it again, landed the note-off on the still-releasing first voice (a
no-op) and left the held one stuck, needing a MIDI panic. Two fingers on one
key hit the same trap.

Give voices an isGated flag (held = ADSR not in release/idle) and make
auditionSlotOff / liveNoteOff release a gated voice of that pitch in preference
to one already ringing out. Add SynthVoiceGateTest covering the gate/release
distinction and the exact stuck-note scenario.

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 = 23
-        versionName = "0.8.0"
+        versionCode = 24
+        versionName = "0.8.1"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"