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.

src/core/instrument/engine/play_params.h

@@ -195,12 +195,10 @@ bool splineActive(const Play& p) {
            (p.filter.enabled && p.filterSpline.mode == EnvMode::Spline);
 }
 
-// The one enforcement of the rule above: forces `p` to Trigger the instant it goes
-// splineActive. Every path that can leave `p` in a state splineActive() would newly return true
-// for — resolving stored params toward the engine, or an editor control edit that can flip a
-// mode/enable toggle — routes through this, so none of them can reopen the Gate+spline hole
-// splineActive's own doc comment describes. Header-inline and allocation-free: play_params.h
-// sits on the per-voice-per-sample include path.
+// The one enforcement of splineActive's rule (see its doc above). Header-inline and
+// allocation-free: play_params.h sits on the per-voice-per-sample include path. Both callers —
+// resolvePlay (sample_map.cpp) and the editor's applyControl — route through here, so the two
+// cannot drift apart.
 template <class Play>
 void enforceGateUnavailableWhileDrawn(Play& p) {
     if (splineActive(p)) p.playMode = PlayMode::Trigger;