instrument: latch the note done at the read-head run-off, fit the migrated fades, and lift the curve dial and overlay selection into pure modules

src/core/instrument/ui/deck_groups.cpp

@@ -191,6 +191,32 @@ bool isLiveDeckParam(DeckParam id) {
     return false;  // unreachable for a valid enumerator; silences a warning.
 }
 
+OverlayEnv overlayEnvForRadio(int radioId) {
+    switch (static_cast<DeckParam>(radioId)) {
+        case DeckParam::kAmpEnvSelect:    return OverlayEnv::kAmp;
+        case DeckParam::kPitchEnvSelect:  return OverlayEnv::kPitch;
+        case DeckParam::kFilterEnvSelect: return OverlayEnv::kFilter;
+        default:                          return OverlayEnv::kNone;
+    }
+}
+
+OverlayEnv nextOverlaySelection(OverlayEnv current, int radioId) {
+    const OverlayEnv picked = overlayEnvForRadio(radioId);
+    if (picked == OverlayEnv::kNone) return current;  // not a radio: nothing selects
+    return (current == picked) ? OverlayEnv::kNone : picked;
+}
+
+bool overlayEnvInert(OverlayEnv env, bool pitchEnvEnabled, bool filterEnabled) {
+    switch (env) {
+        case OverlayEnv::kPitch:  return !pitchEnvEnabled;
+        case OverlayEnv::kFilter: return !filterEnabled;
+        case OverlayEnv::kAmp:
+        case OverlayEnv::kNone:
+            return false;
+    }
+    return false;  // unreachable for a valid enumerator; silences a warning.
+}
+
 bool liveCommitFor(LiveDragKind kind, int paramId) {
     switch (kind) {
         case LiveDragKind::kDeckKnob:

src/core/instrument/ui/deck_groups.h

@@ -133,6 +133,25 @@ enum class LiveDragKind { kOther, kDeckKnob, kEnvNode };
 // can reach — AHDSR or AHD, on any of the three envelopes — is itself live.
 bool liveCommitFor(LiveDragKind kind, int paramId);
 
+// Which envelope the waveform overlay draws and edits. Exclusive across the three envelope
+// decks, and kNone is a valid resting state — the editor opens there. Transient view state:
+// never persisted, never a parameter.
+enum class OverlayEnv { kNone, kAmp, kPitch, kFilter };
+
+// The envelope a deck's overlay-select radio picks; kNone for any other control id.
+OverlayEnv overlayEnvForRadio(int radioId);
+
+// The selection a click on `radioId` produces from `current`. Two rules, provable here rather
+// than in the shell: picking another deck's radio switches to it (exclusivity), and clicking
+// the ACTIVE one clears back to kNone — "no envelope shown" is a state the user can get back
+// to, not an error. A non-radio id leaves the selection alone.
+OverlayEnv nextOverlaySelection(OverlayEnv current, int radioId);
+
+// Whether the overlay for `env` is INERT: its deck group's enable toggle is off, so its knobs
+// are drawn-but-dead and a node drag on the same params must be too — otherwise a drag reaches
+// a param a knob couldn't (envelope_edit.h). Amp has no enable toggle and is never inert.
+bool overlayEnvInert(OverlayEnv env, bool pitchEnvEnabled, bool filterEnabled);
+
 // The deck's BIPOLAR knob law: 0.5 of the knob's travel is zero depth, the ends are -1 and
 // +1. Exact inverses, and exact at the centre detent (0.5 -> 0 -> 0.5), so a knob parked at
 // centre can never persist a hair of modulation. Out-of-range norm clamps to the endpoints.