fix: close review findings on spline EGs — engine, codec, and popup/overlay UI grammar

Live pitch depth, Gate/Spline enable-rule agreement, inert kTrigLength, NaN wire guards, hard-flag-tail corruption no longer wipes the record, RT/cold spline tie-break, retired alt-click, marker-shadow fix, plus new test coverage.

src/core/instrument/ui/deck_groups.cpp

@@ -36,9 +36,8 @@ std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode) {
         penv.captionWidth = 58;
         penv.captionRadio = {id(DeckParam::kPitchEnvSelect)};
         penv.captionToggle = {id(DeckParam::kPitchEnvEnable), 32};
-        // The mode toggle rides the caption slack rather than the knob row: every env group's
-        // knob row is wider than its caption row, so this costs no group width — and the deck
-        // has six pixels of headroom on its first row at the editor's floor width.
+        // The mode toggle rides the caption slack rather than the knob row — costs no group
+        // width; see this module's CLAUDE.md bullet (knob_deck) for the headroom this relies on.
         penv.captionToggle2 = {id(DeckParam::kPitchEnvMode), kEnvModeSegW};
         penv.cellIds = {id(DeckParam::kPitchEnvAttack),
                         id(DeckParam::kPitchEnvHold),
@@ -289,6 +288,11 @@ bool deckKnobInert(DeckParam id, const DeckEnableState& state) {
         case DeckParam::kTrigHold:
         case DeckParam::kTrigDecay:
             return state.ampSpline;
+        // The Trigger %-length knob is inert whenever ANY spline is active (not just the amp's):
+        // a drawn contour always covers the full sample length (splineActive, play_params.h), so
+        // the engine ignores lengthFraction in that case regardless of which EG is drawn.
+        case DeckParam::kTrigLength:
+            return state.ampSpline || state.pitchSpline || state.filterSpline;
         case DeckParam::kPitchEnvAttack:
         case DeckParam::kPitchEnvHold:
         case DeckParam::kPitchEnvDecay:

src/core/instrument/ui/knob_deck.h

@@ -65,9 +65,8 @@ struct DeckGroupDesc {
     DeckRadioDesc captionRadio;    // the caption row's far corner; id -1 = none
     DeckToggleDesc captionToggle;  // caption row, left of the radio; id -1 = none
     // A second caption toggle, placed immediately left of the first (or in its place when the
-    // first is absent). Exists because a group whose knob row is wider than its caption row has
-    // caption slack a toggle can occupy for free — a rowToggle would widen the GROUP, and the
-    // deck has six pixels of headroom on its first row at the editor's floor width.
+    // first is absent) — why this exists rather than a rowToggle is recorded once, at this
+    // module's CLAUDE.md bullet.
     DeckToggleDesc captionToggle2;
     std::vector<int> cellIds;      // knob cells; -1 = blank reserve
     DeckToggleDesc rowToggle;      // in the knob row after the cells; id -1 = none