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_deck_groups.cpp

@@ -619,14 +619,14 @@ static bool sameLayout(const DeckLayout& a, const DeckLayout& b) {
 
 // A Spline excursion is fully reversible at the layout level: the mode forcing swaps the amp
 // and filter faces onto their wider cells and back, leaving no residue in the geometry. Driven
-// through splineActive and the editor's own forcing rule, so the deck cannot agree with a
-// forcing rule the shell does not use.
+// through the shared enforceGateUnavailableWhileDrawn helper, so the deck cannot agree with a
+// forcing rule the real callers do not use.
 static void testGateSplineGateRoundTripsToTheSameLayout() {
     PlayParams p;  // Gate, all three envelopes staged
     const DeckLayout before = layoutDeck(sampleDeckGroups(p.playMode), kPad, 0, kAvailAtMinWidth);
 
     p.ampSpline.mode = EnvMode::Spline;
-    if (splineActive(p)) p.playMode = PlayMode::Trigger;  // editor_controls' forcing, verbatim
+    enforceGateUnavailableWhileDrawn(p);  // the shared helper both real callers route through
     CHECK(p.playMode == PlayMode::Trigger);
     const DeckLayout drawn = layoutDeck(sampleDeckGroups(p.playMode), kPad, 0, kAvailAtMinWidth);
     // The excursion is real: the amp face's cells are strictly wider than Gate's.