Fix stuck audition notes under rapid / chord tapping (v0.8.1)

The synth/SF2 audition note-off matched a voice purely by pitch, and isActive
stays true through a voice's release tail. So tapping a key, then immediately
holding it again, landed the note-off on the still-releasing first voice (a
no-op) and left the held one stuck, needing a MIDI panic. Two fingers on one
key hit the same trap.

Give voices an isGated flag (held = ADSR not in release/idle) and make
auditionSlotOff / liveNoteOff release a gated voice of that pitch in preference
to one already ringing out. Add SynthVoiceGateTest covering the gate/release
distinction and the exact stuck-note scenario.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/test/java/space/rcmd/android/sizzle/audio/SynthVoiceGateTest.kt

@@ -0,0 +1,73 @@
+// SPDX-FileCopyrightText: 2026 Reactorcoremeltdown
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package space.rcmd.android.sizzle.audio
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * Locks the gate-vs-release distinction the audition note-off relies on. A voice in
+ * its release tail must report [SynthVoice.isActive] == true (still sounding) but
+ * [SynthVoice.isGated] == false, so a note-off preferring a gated voice never lands
+ * on a releasing one — the fix for stuck notes under rapid / chord tapping.
+ */
+class SynthVoiceGateTest {
+
+    private val sr = 48_000
+
+    @Test fun heldNoteIsGated() {
+        val v = SynthVoice(sr)
+        v.noteOn(60, 100, SynthPatch.DEFAULT)
+        assertTrue("a freshly triggered note is gated", v.isGated)
+        assertTrue(v.isActive)
+        // Advance well past the attack/decay into sustain — still held.
+        repeat(sr / 2) { v.render() }
+        assertTrue("a sustaining note is still gated", v.isGated)
+        assertTrue(v.isActive)
+    }
+
+    @Test fun releasingNoteIsActiveButNotGated() {
+        val v = SynthVoice(sr)
+        v.noteOn(60, 100, SynthPatch.DEFAULT)
+        repeat(sr / 4) { v.render() } // reach sustain
+        v.noteOff()
+        // One sample into the release: still audible, but no longer gated.
+        v.render()
+        assertTrue("a releasing voice is still active (audible tail)", v.isActive)
+        assertFalse("a releasing voice is no longer gated", v.isGated)
+    }
+
+    @Test fun releaseReachesSilence() {
+        val v = SynthVoice(sr)
+        v.noteOn(60, 100, SynthPatch.DEFAULT)
+        repeat(sr / 4) { v.render() }
+        v.noteOff()
+        // DEFAULT release is 0.2 s; render a full second to be safe.
+        repeat(sr) { v.render() }
+        assertFalse("the voice frees itself after the release completes", v.isActive)
+        assertFalse(v.isGated)
+    }
+
+    /** The stuck-note scenario at the voice level: a releasing voice and a held voice
+     *  of the same pitch coexist; selecting the gated one for note-off leaves no held
+     *  voice behind (whereas picking the first *active* match would strand it). */
+    @Test fun gatedPreferenceReleasesTheHeldVoice() {
+        val releasing = SynthVoice(sr)
+        val held = SynthVoice(sr)
+        releasing.noteOn(60, 100, SynthPatch.DEFAULT)
+        repeat(sr / 4) { releasing.render() }
+        releasing.noteOff()
+        releasing.render() // now active-but-not-gated
+        held.noteOn(60, 100, SynthPatch.DEFAULT) // same pitch, gated
+
+        val pool = listOf(releasing, held)
+        // Mirror auditionSlotOff's selection: prefer a gated voice of this pitch.
+        val target = pool.firstOrNull { it.pitch == 60 && it.isGated }
+            ?: pool.firstOrNull { it.isActive && it.pitch == 60 }
+        target?.noteOff()
+
+        assertFalse("the held voice received the note-off and is releasing", held.isGated)
+    }
+}