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

src/core/instrument/ui/sample_bands.h

@@ -48,6 +48,12 @@ struct WaveformLanes {
     Rect upper;
     Rect lower;  // empty() in mono
 };
-WaveformLanes waveformLanes(const Rect& waveform, bool stereo);
+
+// A RESOLVED lane-split decision, not "is the instrument in stereo mode" — a mono source
+// stays Single even in stereo mode (dual-mono, no second channel to draw). Only
+// waveformSurface (waveform_view) folds the source channel count in; a bare bool here would
+// let a caller pass isStereoMode straight through and skip that check.
+enum class LaneSplit { Single, Stereo };
+WaveformLanes waveformLanes(const Rect& waveform, LaneSplit split);
 
 }  // namespace reasampler::instrument::ui