`compose::flatten` returns an empty buffer for canvases over MAX_RENDER_PIXELS.
Trim (Top Left) indexed `px[0]` of it and Duplicate › Merged Layers Only fed it
to `write_region`'s length assert: both panicked; Trim's other modes reported
"nothing to trim to". Both now return a "too large to flatten" error first.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
