fix(voice): CC 120 hard-stops voices (incl. Trigger); Mono sizes voices_ to 1; comments corrected

CC 120 -> allSoundsOff/hardStop (immediate silence, stops Trigger one-shots); CC 123 -> allNotesOff/releaseAll (release, unchanged). Mono VoiceEngine sizes voices_ to 1 structurally. Legato same-note re-press edge documented. Three new tests.

src/vst/sampler_core.h

@@ -428,6 +428,11 @@ public:
     // TRIGGER mode it is a NO-OP (Trigger ignores note-off and plays through to its play length).
     void release();
 
+    // HARD STOP — CC 120 (All Sounds Off) semantics. Immediately silences this voice regardless
+    // of play mode: sets active_ = false with no release ramp. Stops a ringing Trigger one-shot
+    // instantly (which release() cannot do). RT-safe: no allocation, no lock.
+    void hardStop();
+
     // True while this voice is producing (or about to produce) sound.
     bool active() const { return active_; }
     // The note this voice was started on (for note-off routing). Meaningless if idle.
@@ -568,14 +573,19 @@ public:
     // the older tail to ring — matches hardware behavior). No-op if none match.
     void noteOff(int note);
 
-    // PANIC / MIDI All-Notes-Off (CC 123). Clears the MONO held stack and releases EVERY
-    // active voice: Gate voices enter their release tail; Trigger one-shots ignore release
-    // by design and play through their bounded play length (they can never be stuck). This
-    // is the mono stack's ONLY reset path — a phantom entry left by a lost note-off would
-    // otherwise be resurrected by the next fallback and sustain forever with no key held.
-    // RT-safe (no allocation, bounded by maxVoices); callable from the audio thread.
+    // CC 123 — MIDI All-Notes-Off: clears the MONO held stack and RELEASES every active voice
+    // (Gate voices enter their AHDSR release tail; Trigger one-shots ignore release and play
+    // through their bounded play length). This is the mono stack's ONLY reset path — a phantom
+    // entry left by a lost note-off would otherwise be resurrected by the fallback and sustain
+    // forever with no key held. RT-safe (no allocation, bounded by maxVoices).
     void allNotesOff();
 
+    // CC 120 — MIDI All-Sounds-Off: hard-stops EVERY voice immediately (active_ = false, no
+    // release ramp), clears the MONO held stack, and silences even Trigger one-shots that would
+    // ignore a release. Use for panic; CC 123 for the softer "let gates release" behavior.
+    // RT-safe (no allocation, bounded by maxVoices); callable from the audio thread.
+    void allSoundsOff();
+
     // REAL-TIME render (S4): sums all active voices into the caller-provided buffer
     // `out[0..frameCount)`, ADDING to whatever is there (the caller clears or mixes —
     // this never touches memory it does not own and NEVER allocates). This is the
@@ -680,9 +690,12 @@ public:
     // Release the preview IF `note` is the one sounding (a stale off for a replaced note is a
     // no-op). Gate zones enter release; Trigger zones ignore note-off and play through.
     void noteOff(int note);
-    // PANIC peer of VoiceEngine::allNotesOff: release the ringing preview UNCONDITIONALLY,
+    // CC 123 peer of VoiceEngine::allNotesOff: release the ringing preview UNCONDITIONALLY,
     // whatever note it is on. Gate enters release; Trigger plays through (bounded). RT-safe.
     void releaseAll();
+    // CC 120 peer of VoiceEngine::allSoundsOff: HARD-STOP the preview immediately (no release
+    // ramp, silences Trigger one-shots too). RT-safe.
+    void hardStop();
 
     bool active() const { return voice_.active(); }