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/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