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

src/shell/instrument/reasampler_editor.h

@@ -45,6 +45,7 @@ using instrument::ui::ChromeRects;
 using instrument::ui::DeckGroupDesc;
 using instrument::ui::EnvClampBounds;
 using instrument::ui::EnvNode;
+using instrument::ui::OverlayArea;
 using instrument::ui::Rect;
 using instrument::ui::SampleBands;
 
@@ -173,7 +174,7 @@ private:
     void paintVelocityCurve(LICE_IBitmap* bmp, const Rect& r);
     // Traces the amp-envelope overlay + its draggable node handles over `waveArea`, ONCE at
     // full band height (never per lane).
-    void paintEnvelopeOverlay(LICE_IBitmap* bmp, const Rect& waveArea, std::int64_t frames);
+    void paintEnvelopeOverlay(LICE_IBitmap* bmp, const OverlayArea& waveArea, std::int64_t frames);
 
     // --- Input: the mouse-down dispatch and its per-band branches ---
     void onMouseDown(int x, int y);