Scale-filter toolbox keyboards; pin sampler pads at keyboard height (v0.7.2)

- Scale/root filtering on the Toolbox audition keyboards, matching the tracker
  punch-in keyboard: out-of-scale keys are dimmed and inert (Chromatic = all
  playable). StackedKeyboard now filters in both punch-in and audition mode,
  and SlotAuditionKeyboard (synth / SF2 editors) gained the same filter.
- Sampler editor restructured so the 16-pad grid is pinned at the bottom at the
  same height the audition keyboards use, consistent across every instrument
  editor. The per-pad volume faders moved to a second swipe page (waveform on
  page 1, volumes on page 2, with dot pagination), and pad cells now fill the
  pinned region instead of a fixed 46dp. Selected-pad state is hoisted into
  ParamEditor so the detail area and the pinned grid stay in sync.

Verified on-device: audition keyboard dims to F harmonic minor; sampler pads
fill the pinned bottom region with volumes on the swipe page.

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 = 20
-        versionName = "0.7.1"
+        versionCode = 21
+        versionName = "0.7.2"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"