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

src/core/instrument/ui/envelope_overlay.h

@@ -93,8 +93,9 @@ double gatePxPerSecond(const Rect& area);
 // Gate's x-axis is a bounded schematic independent of totalSeconds (does NOT line up with the
 // waveform under it); Trigger's x-axis is PCM-aligned wall-clock. Every vertex is clamped inside
 // the canvas: x in [area.x, area.right()-1], y in [area.y, area.bottom()-1]. A degenerate area
-// or totalSeconds <= 0 yields the flat two-point baseline [Origin, end at level 0].
-std::vector<EnvVertex> buildEnvelopePolyline(const AmpEnvelope& env, const Rect& area,
+// or totalSeconds <= 0 yields the flat two-point baseline [Origin, end at level 0]. Takes the
+// waveform overlay (not a lane) — see waveform_view.h's overlay contract.
+std::vector<EnvVertex> buildEnvelopePolyline(const AmpEnvelope& env, const OverlayArea& area,
                                              double totalSeconds);
 
 // Maps a time (seconds) to a pixel x inside `area`, linear and clamped at both ends. Shared