instrument: snap live params onto a fresh voice, roll a live drag back on capture loss, serialize the seqlock's two writers

src/core/instrument/ui/deck_groups.h

@@ -77,15 +77,35 @@ std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode);
 // controls, so this is a routing decision at the editor's commit site rather than a property
 // of any one knob; moving a control across the line is a change here and nowhere else.
 //
-// Deliberately NOT live, each for its own reason: the discrete toggles (play mode, pitch
-// engine, filter enable/law, pitch-envelope enable) name a different sound rather than a
-// different setting of one; the three capture-anchored overrides (root, loop span, start
-// frame) name positions in the decoded PCM; pitch key-track and the velocity->cutoff depth
-// feed values a voice latches at note-on by design (the pitch ratio and the velocity-curve
-// result), so making them live would retune or re-gain a note already struck; and Trigger's
-// %-length and fades resolve a play span that is a fact about the note.
+// THE home for why each excluded control is excluded. Five continuous controls are outside the
+// live set, plus every discrete toggle:
+//   - the discrete toggles (play mode, pitch engine, filter enable/law, pitch-envelope enable)
+//     name a different sound rather than a different setting of one;
+//   - the three capture-anchored overrides (root, loop span, start frame) name positions in
+//     the decoded PCM;
+//   - kKeyTrack and kFilterVel feed values a voice latches at note-on by design (the pitch
+//     ratio and the velocity-curve result), so live delivery would retune or re-gain a note
+//     already struck;
+//   - kTrigLength resolves playEnd_, a fact about the note. kTrigFadeIn/kTrigFadeOut are pure
+//     amplitude shape and would be live-able in principle, but they live in `sample.play` and
+//     are baked into SampleData at build time — the engine rebuild copies that verbatim, so
+//     only a reload can deliver them without widening LiveValues. They fold into the AHD
+//     alongside Gate's, at which point they inherit its routing; until then they reload.
+// Consequence, stated plainly: a Trigger-mode instance gets NO live delivery on its amplitude
+// controls. Only the filter and pitch-envelope knobs move a sounding Trigger one-shot.
 bool isLiveDeckParam(DeckParam id);
 
+// The editor drag kinds that can commit live, in this pure module's own vocabulary (the
+// shell's DragKind maps onto it) so the WHOLE routing decision — not just the predicate — is
+// testable without a host.
+enum class LiveDragKind { kOther, kDeckKnob, kEnvNode };
+
+// Whether a drag of `kind` commits live. A deck knob is live per isLiveDeckParam (negative ids
+// are the shell's processor-side sentinels and out-of-range ids are not controls, so neither
+// reaches the enum); an envelope-node drag is live only in Gate, where it edits the AHDSR —
+// in Trigger the same drag rewrites the play span, which is not a live control.
+bool liveCommitFor(LiveDragKind kind, int paramId, PlayMode playMode);
+
 // 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.