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/velocity_curve.h

@@ -232,13 +232,14 @@ public:
     // True once the cursor has settled on the contour's LAST segment. On its own this does NOT
     // make a 0 read here a terminus: the final segment's LEFT endpoint can also be 0 (a 2-point
     // contour is nothing but a single "final" segment starting at frame 0), which would read 0
-    // while about to rise. Voice::tickAmplitude pairs this with terminalValue() == 0 — the
+    // while about to rise. Voice::tickAmplitude pairs this with segmentEndValue() == 0 — the
     // segment's RIGHT endpoint, i.e. the whole contour's true end — before calling a 0 read the
     // note's genuine permanent terminus.
     bool onFinalSegment() const { return seg_ + 2 == n_; }
-    // The current segment's right endpoint — the whole contour's terminal Y only when paired
-    // with onFinalSegment() (see there).
-    double terminalValue() const { return y1_; }
+    // The CURRENT SEGMENT's right endpoint — not a contour-level concept despite the name's
+    // shape; it is the whole contour's terminal Y only when paired with onFinalSegment() (see
+    // there). Named for what it returns, not for its one call site's use of it.
+    double segmentEndValue() const { return y1_; }
 
     // `phase` is normalized position over the contour's whole span, [0,1]; out-of-range clamps
     // to the terminal values (a note past its span holds the contour's last level).