Before decoding, loadImages reads every header to size the batch against
the free memory. That pass was serial, silent and could not be cancelled,
so the loading window sat at "0 of N". LoadProgress gains onSized, the
headers are read in parallel on the same cursor-driven workers as the
decodes, and a cancel stops the pass. The window's bar runs twice,
"Checking sizes" then "Decoding", and the CLI prints a sizing line.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
