Three combined additions to the corrected-image viewer:
- Circle measurement tool: 2-click placement (center + edge), dedicated
hit-test, handle drag preserving radius when grabbing the center.
- Annotated PNG exports via two new buttons in the result page:
"Download full + measurements" (source resolution, strokes scaled up
to read at the same visual weight) and "Download view + measurements"
(current pan/zoom). Both respect the existing scale-bar toggle; the
view export's bar is sized for canvas-px/mm = image-px/mm × view scale.
- Per-image measurement persistence keyed by file hash, mirroring the
datum cache. "Clear cache" in the upload step now wipes both.
Drawing helpers were refactored to take a RenderCtx (transform +
strokeMul + handle/decoration flags) so the same code paths handle live
overlay and offscreen export.
