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/voice.cpp

@@ -102,8 +102,12 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
         playEnd_ = 0;  // unused in Gate
     } else {
         // Trigger: play [start, playEnd) where
-        // playEnd = start + round(lengthFraction*(frames-start)).
-        double frac = p.trigger.lengthFraction;
+        // playEnd = start + round(lengthFraction*(frames-start)) — except kTrigLength is INERT
+        // while any spline EG is active (splineActive, play_params.h): a contour is a pure
+        // function over the FULL sample length, so truncating playEnd_ to a %-length would
+        // hard-cut it mid-shape. The staged Trigger AHD below (trigSpan) plays the same full
+        // span in that case, matching the "drawn-but-dead" treatment of the other staged knobs.
+        double frac = splineActive(p) ? 1.0 : p.trigger.lengthFraction;
         if (frac <= 0.0) frac = 0.0;      // %=0 -> zero play length (finishes immediately)
         if (frac > 1.0) frac = 1.0;
         std::int64_t playLen = static_cast<std::int64_t>(
@@ -258,6 +262,12 @@ void Voice::applyLive(const instrument::engine::LiveValues& live, bool snap) {
         else ampAhd_.applyLive(sourceOffset(), live.ampAhd);
         pitchEnv_.applyLive(live.pitchEnv);
     }
+    // The pitch DEPTH knob stays live under a spline (core/instrument/CLAUDE.md), but
+    // pitchSplineDepth_ is a plain member latched at note-on — unlike filter's modAmount_,
+    // which already glides through rModAmount_'s live ramp regardless of spline state (below),
+    // this is the one place a live pitch-depth move must be re-applied by hand. Only meaningful
+    // while pitchSplineCur_ is bound; harmless (and cheap) to set otherwise.
+    pitchSplineDepth_ = live.pitchEnv.peakSemitones;
     if (!filterOn_) return;  // filter enable is a discrete toggle: it travels by reload
 
     if (snap) {