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/velocity_curve.h

@@ -229,11 +229,16 @@ public:
     }
     void clear() { pts_ = nullptr; n_ = 0; }
     bool active() const { return n_ >= 2; }
-    // True once the cursor has settled on the contour's LAST segment: past this point there is
-    // no further point to rise into, so a value read here that reaches 0 is a genuine permanent
-    // terminus (Voice::tickAmplitude's early-free), unlike a 0 touched mid-contour, which a
-    // later segment may still rise out of (the spline is deliberately not globally monotone).
+    // True once the cursor has settled on the contour's LAST segment. On its own this does NOT
+    // make a 0 read here a terminus: the final segment's LEFT endpoint can also be 0 (a 2-point
+    // contour is nothing but a single "final" segment starting at frame 0), which would read 0
+    // while about to rise. Voice::tickAmplitude pairs this with terminalValue() == 0 — the
+    // segment's RIGHT endpoint, i.e. the whole contour's true end — before calling a 0 read the
+    // note's genuine permanent terminus.
     bool onFinalSegment() const { return seg_ + 2 == n_; }
+    // The current segment's right endpoint — the whole contour's terminal Y only when paired
+    // with onFinalSegment() (see there).
+    double terminalValue() const { return y1_; }
 
     // `phase` is normalized position over the contour's whole span, [0,1]; out-of-range clamps
     // to the terminal values (a note past its span holds the contour's last level).