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).

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;

src/core/instrument/ui/knob_deck.cpp

@@ -180,11 +180,12 @@ DeckHit hitTestDeck(const DeckLayout& layout, int x, int y) {
                 return {DeckHitKind::RowToggle, g.rowToggle.id, 1};
         }
         for (const DeckCellLayout& c : g.cells) {
-            if (c.id >= 0 && contains(c.cell, x, y)) {
+            // Every entry here already has a real id — a reserve yields no DeckCellLayout at all.
+            if (contains(c.cell, x, y)) {
                 return {DeckHitKind::Knob, c.id, -1, contains(c.inner, x, y)};
             }
         }
-        return {};  // inside the box but on fence/padding/blank — a miss (groups never overlap)
+        return {};  // inside the box but on fence/padding — a miss (groups never overlap)
     }
     return {};
 }

src/core/instrument/ui/knob_deck.h

@@ -69,7 +69,7 @@ struct DeckGroupDesc {
     // first is absent) — why this exists rather than a rowToggle is recorded once, at this
     // module's CLAUDE.md bullet.
     DeckToggleDesc captionToggle2;
-    std::vector<int> cellIds;      // knob cells; -1 = blank reserve
+    std::vector<int> cellIds;      // knob cells; -1 reserves width only, no cell (see above)
     DeckToggleDesc rowToggle;      // in the knob row after the cells; id -1 = none
 };
 

src/core/instrument/ui/spline_edit.h

@@ -1,7 +1,8 @@
-// spline_edit.h — THE point-editing grammar, and the one place it is written down. Both spline
-// consumers route their mouse-down through it — the velocity-curve popup and the spline EG
-// overlay — so the two cannot drift into two grammars. Mirror of envelope_edit: decision logic
-// only, no host types, no drawing.
+// spline_edit.h — the point-editing grammar's CLICK resolution: add/grab/delete/toggle from a
+// single (x, y). Both spline consumers — the velocity-curve popup and the spline EG overlay —
+// route their mouse-down through it, so the two cannot drift into two click grammars. Two more
+// gesture rules complete the grammar but live in the shell (see the note near the bottom of this
+// file). Mirror of envelope_edit otherwise: decision logic only, no host types, no drawing.
 
 #pragma once
 
@@ -39,4 +40,15 @@ SplineEdit resolveSplineEdit(const VelocityCurve& curve, const VelocityCurve::Bo
 // waveform beneath it. Takes the overlay (not a lane) — see waveform_view.h's overlay contract.
 VelocityCurve::Box splineOverlayBox(const OverlayArea& area);
 
+// Two more rules complete the grammar. Both are enforced in the shell — mouse-tracking / drag
+// state has no home in a pure module — and recorded here as their one home rather than restated
+// at each call site:
+// - DRAG-OFF DELETE: releasing a grabbed node well outside its box deletes it (endpoints exempt,
+//   per deletePoint's own refusal) — editor_input.cpp's onMouseUp, shared verbatim by the popup
+//   and the overlay.
+// - THE OVERLAY'S OUTSIDE-BOX EXCEPTION: resolveSplineEdit's own outside-box grab/toggle/delete
+//   allowance (above) is meant for the popup's inset ring; the overlay narrows it back to
+//   strictly in-box, since splineOverlayBox has no inset — editor_input_waveform.cpp's
+//   splineOverlayClick.
+
 }  // namespace reasampler::instrument::ui