Fix synth crashes, dead filter, and playback hiccups (v0.4.3)

Address three issues introduced with the 3-oscillator synth:

- Crashes: ToolboxSlot.values was a plain HashMap read on the audio thread while
  the UI wrote it (knob drags vs. the synth's ~40-param-per-note reads). Made it a
  ConcurrentHashMap so get/put are atomic and reads are lock-free (RT-safe).
- Filter cutoff not affecting the sound: (a) the cheap ladder went unstable at high
  cutoff (f > 1 -> NaN -> silent voice) — replaced with a stable Cytomic TPT
  state-variable low-pass (with NaN recovery); (b) a voice captured its patch only
  at note-on, so live knob turns were inaudible — the engine now refreshes active
  voices' patches every block, so cutoff/level/mod changes affect held & playing
  notes.
- Hiccups: every note allocated a SynthPatch (+arrays) causing GC dropouts — the
  patch is now cached per bus and reparsed only on param/tempo change, so steady
  playback allocates nothing on the audio thread.

Patch bump to 0.4.3 (versionCode 10).

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 = 9
-        versionName = "0.4.2"
+        versionCode = 10
+        versionName = "0.4.3"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"