fix: close round-2 review findings — Critical silent-note bug plus majors/minors

Fixes the 2-point spline-EG early-free bug causing silent fade-ins, the pitch/filter enable-toggle Trigger-forcing hole, the contour-node/marker pixel shadow, missing deck-residue test coverage, and stale comments in knob_deck and spline_edit.

src/core/instrument/engine/voice.h

@@ -186,12 +186,18 @@ private:
         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. 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
+            // whose TERMINAL value (the final segment's right endpoint) is 0 reaches a genuine
+            // permanent terminus early, the spline analogue of a staged AHD's finished(). Gating
+            // on the terminal value, not just the segment index, matters because a 2-point
+            // contour IS a single "final" segment from frame 0 — checking onFinalSegment() alone
+            // would call a contour that STARTS at 0 (e.g. a fade-in) over before it ever rises.
+            // Mid-contour dips through 0 still don't free early, since the spline is deliberately
             // not globally monotone.
             amp = ampSplineCur_.eval(splinePhase());
-            if (amp == 0.0 && ampSplineCur_.onFinalSegment()) amplitudeDone_ = true;
+            if (amp == 0.0 && ampSplineCur_.onFinalSegment() &&
+                ampSplineCur_.terminalValue() == 0.0) {
+                amplitudeDone_ = true;
+            }
         } else if (playMode_ == PlayMode::Gate) {
             amp = env_.tick();
             if (env_.finished()) amplitudeDone_ = true;