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/engine/play_params.h

@@ -183,10 +183,16 @@ struct PlayParams {
 // sample length, which IS the Trigger/one-shot playback model — so Gate is not available while
 // any spline EG is active. resolvePlay enforces it on the way to the engine; the editor's
 // play-mode toggle refuses the Gate segment so the two agree.
+//
+// The pitch/filter terms are gated on their own `enabled` flag to match Voice::start's binder
+// (voice.cpp only binds pitchSplineCur_/filterSplineCur_ when that flag is set): without this,
+// a Spline mode flip on a disabled pitch/filter envelope would cost Gate for zero modulation,
+// since the binder would never actually engage. Amp has no such flag, so it counts unconditionally.
 template <class Play>
 bool splineActive(const Play& p) {
-    return p.ampSpline.mode == EnvMode::Spline || p.pitchSpline.mode == EnvMode::Spline ||
-           p.filterSpline.mode == EnvMode::Spline;
+    return p.ampSpline.mode == EnvMode::Spline ||
+           (p.pitchEnv.enabled && p.pitchSpline.mode == EnvMode::Spline) ||
+           (p.filter.enabled && p.filterSpline.mode == EnvMode::Spline);
 }
 
 // [start, end) frames, half-open. A zero-length loop (start == end) is the "no sustain loop"