Pin the SF2/XI audition keyboard at the editor bottom (v0.3.1)

Lay out and place the SF2/XI loader's audition keyboard like the toolbox and
tracker keyboards: extract it into SoundFontKeyboard (an octave selector above two
weighted octaves) and pin it at the bottom of the device editor, with the controls
scrolling in a weighted region above (1.7 : 1, matching the toolbox tab) — so it's
the same height and always visible instead of scrolling away. Patch bump to 0.3.1
(versionCode 4).

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 = 3
-        versionName = "0.3.0"
+        versionCode = 4
+        versionName = "0.3.1"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"