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/map/sample_map.cpp

@@ -262,10 +262,10 @@ PlayParams resolvePlay(const PlaySeconds& stored, int sampleRate) {
     out.ampSpline    = stored.ampSpline;
     out.pitchSpline  = stored.pitchSpline;
     out.filterSpline = stored.filterSpline;
-    // Gate is unavailable while any EG is drawn — see splineActive (play_params.h) for why.
-    // The editor refuses the Gate segment for the same reason; enforcing it HERE as well is
-    // what keeps a hand-edited or downgraded blob from reaching the engine as Gate + spline.
-    if (splineActive(stored)) out.playMode = PlayMode::Trigger;
+    // Every field splineActive reads on `out` is already copied from `stored` above, so this
+    // enforces the same rule enforceGateUnavailableWhileDrawn's doc comment (play_params.h)
+    // describes — the editor's applyControl is the other caller, so the two cannot drift.
+    enforceGateUnavailableWhileDrawn(out);
     return out;
 }