Type-enforce the waveform overlay contract, narrow waveformLanes to LaneSplit, fix laneCount/cache/path-fallback bugs

src/core/instrument/ui/envelope_edit.h

@@ -54,7 +54,9 @@ struct NodeHit {
     bool    hit  = false;
     EnvNode node = EnvNode::Origin;  // meaningful only when hit == true
 };
-NodeHit nodeAtPoint(const AmpEnvelope& env, const Rect& area, double totalSeconds, int x, int y);
+// Takes the waveform overlay (not a lane) — see waveform_view.h's overlay contract.
+NodeHit nodeAtPoint(const AmpEnvelope& env, const OverlayArea& area, double totalSeconds, int x,
+                    int y);
 
 // Resolves a drag of `node` to a new AmpEnvelope. `grabEnv` is the envelope as of grab time (the
 // shell snapshots it on button-down so the delta is absolute, not accumulated); `dxPixels`/
@@ -65,7 +67,7 @@ NodeHit nodeAtPoint(const AmpEnvelope& env, const Rect& area, double totalSecond
 //   * A non-draggable node, an other-mode node, a zero-size area, or totalSeconds <= 0 returns
 //     `grabEnv` unchanged.
 // Only the dragged node's param(s) change. Pure.
-AmpEnvelope resolveNodeDrag(const AmpEnvelope& grabEnv, EnvNode node, const Rect& area,
+AmpEnvelope resolveNodeDrag(const AmpEnvelope& grabEnv, EnvNode node, const OverlayArea& area,
                             double totalSeconds, const EnvClampBounds& bounds,
                             int dxPixels, int dyPixels);