fix: close round-3 review findings — smallest-target-first, residue test fix, extraction

Waveform overlay now resolves node/tab/marker click collisions by target area instead of check order; residue test now uses a distinguishing fixture; Gate-unavailable-while-drawn logic extracted to one pure helper shared by resolvePlay and applyControl.

src/core/instrument/engine/play_params.h

@@ -195,6 +195,17 @@ 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.
+template <class Play>
+void enforceGateUnavailableWhileDrawn(Play& p) {
+    if (splineActive(p)) p.playMode = PlayMode::Trigger;
+}
+
 // [start, end) frames, half-open. A zero-length loop (start == end) is the "no sustain loop"
 // marker — a held note past the sample end goes silent rather than looping a zero span.
 struct SampleLoop {