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/voice.h

@@ -184,18 +184,11 @@ private:
         // entirely — release() then has no envelope to end, and an active sustain loop rings
         // forever.
         if (ampSplineCur_.active() && playMode_ == PlayMode::Trigger) {
-            // A contour covers the sample end to end, so the head leaving the span IS the end of
-            // the note — the exhaustion path in advanceFrame is what frees the voice. A contour
-            // whose TERMINAL value (the final segment's right endpoint) is 0 reaches a genuine
-            // permanent terminus early, the spline analogue of a staged AHD's finished(). Gating
-            // on the terminal value, not just the segment index, matters because a 2-point
-            // contour IS a single "final" segment from frame 0 — checking onFinalSegment() alone
-            // would call a contour that STARTS at 0 (e.g. a fade-in) over before it ever rises.
-            // Mid-contour dips through 0 still don't free early, since the spline is deliberately
-            // not globally monotone.
+            // Early-free at a genuine permanent terminus (the spline analogue of a staged AHD's
+            // finished()) — onFinalSegment()/segmentEndValue()'s own doc comments own the why.
             amp = ampSplineCur_.eval(splinePhase());
             if (amp == 0.0 && ampSplineCur_.onFinalSegment() &&
-                ampSplineCur_.terminalValue() == 0.0) {
+                ampSplineCur_.segmentEndValue() == 0.0) {
                 amplitudeDone_ = true;
             }
         } else if (playMode_ == PlayMode::Gate) {