fix: pin the waveform arbitration in a testable predicate, close round-4 review minors

Extracts resolveWaveformClaim (core/instrument/ui/spline_edit) so the shell's node/tab/marker click resolution is unit-tested directly, not just its input geometry; folds the staged-envelope node into it; fixes comment accuracy, a cost regression, and test fidelity issues.

tests/test_spline_egs.cpp

@@ -363,6 +363,33 @@ static void testFlatZeroFinalSegmentStillFreesTheVoiceEarly() {
     CHECK(!v.soundingNote());             // a genuine permanent terminus, not a mid-contour dip
 }
 
+// The timing, not just the fact: the case above is trivially "early" (a wholly-flat contour
+// frees on frame 0), which can't distinguish "frees early" from "frees at the right frame." This
+// fixture's final segment starts MID-sample, so the free must land there, not at frame 0 and not
+// at the sample's natural end. The breakpoint (phase 0.5495) is deliberately off every sampled
+// frame's exact phase (k/1000), so no sampled frame lands on the segment boundary itself and
+// which segment "owns" that frame is never ambiguous.
+static void testFlatZeroFinalSegmentFreesTheVoiceWhereItBeginsNotAtFrameZero() {
+    const double breakpointPhase = 0.5495;
+    const VelocityCurve contour = VelocityCurve::fromPoints(
+        {{xAt(0.0), 1.0}, {xAt(breakpointPhase), 0.0}, {xAt(1.0), 0.0}}, CurveDomain::Unipolar);
+    const std::size_t frames = 1000;
+    const SampleData s = splineAmpSample(frames, contour);
+
+    Voice v;
+    v.start(60, 100, s);
+    // Frames 0..549 (phase < breakpoint) sit on the declining first segment: still sounding.
+    for (std::size_t i = 0; i < 550; ++i) {
+        v.renderFrame();
+        CHECK(v.soundingNote());
+    }
+    // Frame 550 (phase 0.55) is the first sampled frame past the breakpoint, on the flat-zero
+    // final segment — this is where the early-free fires.
+    const double y550 = v.renderFrame();
+    CHECK(near(y550, 0.0, 1e-9));
+    CHECK(!v.soundingNote());
+}
+
 // --- 9. A fresh spline EG opens on the smooth y = 1 - x ----------------------
 
 static void testAFreshSplineEgDefaultsToTheSmoothDownwardSlope() {
@@ -532,6 +559,7 @@ int main() {
     testAContourReplaysProportionallyOnADifferentLengthSample();
     testTwoPointContourRisingFromZeroSoundsForItsFullSpan();
     testFlatZeroFinalSegmentStillFreesTheVoiceEarly();
+    testFlatZeroFinalSegmentFreesTheVoiceWhereItBeginsNotAtFrameZero();
     testAFreshSplineEgDefaultsToTheSmoothDownwardSlope();
     testGateIsUnavailableWhileASplineEgIsActiveAndReturnsAfterwards();
     testEnforceGateUnavailableWhileDrawnForcesTriggerOnBothRepresentations();