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.h

@@ -178,10 +178,20 @@ private:
     // the voice.
     double tickAmplitude() {
         double amp;
-        if (ampSplineCur_.active()) {
+        // playMode_ is Trigger whenever a spline is genuinely reachable (resolvePlay forces it —
+        // splineActive, play_params.h); the guard is a pure-core defense against a hand-built
+        // SampleData pairing Gate with an amp spline, which would otherwise bypass env_
+        // entirely — release() then has no envelope to end, and an active sustain loop rings
+        // forever.
+        if (ampSplineCur_.active() && playMode_ == PlayMode::Trigger) {
             // A contour covers the sample end to end, so the head leaving the span IS the end of
-            // the note — the exhaustion path in advanceFrame is what frees the voice.
+            // the note — the exhaustion path in advanceFrame is what frees the voice. A contour
+            // that flatlines at 0 across its FINAL segment is a permanent terminus (no later
+            // segment to rise out of), so that case frees early too, the spline analogue of a
+            // staged AHD's finished() — mid-contour dips do not, since the spline is deliberately
+            // not globally monotone.
             amp = ampSplineCur_.eval(splinePhase());
+            if (amp == 0.0 && ampSplineCur_.onFinalSegment()) amplitudeDone_ = true;
         } else if (playMode_ == PlayMode::Gate) {
             amp = env_.tick();
             if (env_.finished()) amplitudeDone_ = true;