Arpeggiate the whole held chord, not just the last note (v0.15.1)

Holding a chord on an arpeggiated channel used to collapse to the last (rightmost)
note and retrigger only that — the other notes were killed by the per-note
releaseBus + single-note arp.start. ChannelArp now holds the whole chord (sorted,
de-duped) and cycles all its notes across the octave range in the chosen mode.

Notes entered on the same line accumulate into one chord via a per-line generation
stamp; a new line starts a fresh chord, and a looping chord keeps its running phase
so it doesn't glitch at the loop point. ChannelArp is made internal to cover the
sequencing with ChannelArpTest.

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 = 39
-        versionName = "0.15.0"
+        versionCode = 40
+        versionName = "0.15.1"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"