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.

tests/test_spline_egs.cpp

@@ -321,6 +321,29 @@ static void testAContourReplaysProportionallyOnADifferentLengthSample() {
     CHECK(a[shortLen / 2] > a[10] + 0.2);
 }
 
+// --- Regression: a 2-point contour starting at 0 is not a terminus at frame 0 -----------
+//
+// onFinalSegment() (seg_+2==n_) is trivially true for a 2-point contour. Gating the amp
+// spline's early-free on that alone reads a contour's OWN opening value as the note's end,
+// so the simplest fade-in (left knot dragged to the box floor) went silent at frame 0. The
+// fix requires the TERMINAL value (the segment's right endpoint) to be 0, not just the
+// segment index.
+static void testTwoPointContourRisingFromZeroSoundsForItsFullSpan() {
+    const VelocityCurve contour =
+        VelocityCurve::fromPoints({{kVelMin, 0.0}, {kVelMax, 1.0}}, CurveDomain::Unipolar);
+    const std::size_t frames = 1000;
+    const SampleData s = splineAmpSample(frames, contour);
+
+    Voice v;
+    v.start(60, 100, s);
+    CHECK(v.soundingNote());              // fresh note: sounding before anything is rendered
+    const double y0 = v.renderFrame();
+    CHECK(near(y0, 0.0, 1e-9));           // the contour's own value at phase 0 IS 0 ...
+    CHECK(v.soundingNote());              // ...but the note itself must not be over yet
+    for (std::size_t i = 1; i < frames / 2; ++i) v.renderFrame();
+    CHECK(v.soundingNote());              // still sounding at the midpoint, rising toward 1
+}
+
 // --- 9. A fresh spline EG opens on the smooth y = 1 - x ----------------------
 
 static void testAFreshSplineEgDefaultsToTheSmoothDownwardSlope() {
@@ -467,6 +490,7 @@ int main() {
     testStagedAndSplineStatesBothSurviveAFlipAndASaveReload();
     testAV12PayloadLoadsWithoutLoss();
     testAContourReplaysProportionallyOnADifferentLengthSample();
+    testTwoPointContourRisingFromZeroSoundsForItsFullSpan();
     testAFreshSplineEgDefaultsToTheSmoothDownwardSlope();
     testGateIsUnavailableWhileASplineEgIsActiveAndReturnsAfterwards();
     testTheVelocityAmpCurveGainsTheToggleAndKeepsItsDelete();