Fix waveform_view comment accuracy, cut duplicate comments, align xToFrame to OverlayArea

tests/test_envelope_overlay.cpp

@@ -26,8 +26,6 @@ static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
     std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
 
-// buildEnvelopePolyline takes the overlay type, not a bare Rect (the distinct-type
-// enforcement in editor_geometry.h) — this wraps a plain test Rect for it.
 static OverlayArea overlayOf(const Rect& r) { return OverlayArea{r}; }
 
 // A comfortable overlay area: 1000px wide, 100px tall, offset so left/top != 0 (catches origin