MIDI-learn: map controller knobs to any toolbox device parameter (v0.21.0)

Every instrument/effect editor gains a "⇄ MIDI" button in its toolbar that opens
a MIDI-learn popup listing the device's adjustable parameters. Tap LEARN on a row,
move a knob/fader on a connected MIDI controller, and its CC binds to that
parameter; the CC's 0..127 sweep then drives the parameter's full range live
(scaled to the spec range, rounded for integer params, nearest choice for enums).

Reuses the existing RawControl path — MidiInput already forwards unbound CCs — so
no parser change was needed. Bindings are stored in the slot's value map under a
reserved `midicc@<param>` key, so they travel with the project/preset and are
thread-safe via the slot's ConcurrentHashMap; one physical control drives one
parameter. Learn is auto-disarmed when the popup or editor closes.

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 = 46
-        versionName = "0.20.0"
+        versionCode = 47
+        versionName = "0.21.0"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"