docs(badapple): record the potrace y-up orientation fix

bad-apple/REPORT.md

@@ -47,6 +47,12 @@ rectangle 10..109 × 20..69 became exactly screen x 20..219. The first version
 was exactly twice as wide; the cause was the OS leaving PSIZE at 2 bpp. The
 primitive now re-asserts PSIZE=1 on every call.
 
+Potrace emits **y-up** GeoJSON; the framebuffer is y-down. The first build was
+vertically mirrored, and the VRAM checker still said EXACT because it consumed
+the same flipped data. `trace()` now flips y before simplification, and
+`test_trace_preserves_vertical_position` pins the orientation. The preview PNGs
+were the visible symptom.
+
 ## Pacing
 
 `wait_vsync()` programs `DPYINT=1`, clears `INTPEND`, and polls the DI bit at