Detects truncated/incomplete downloads by checking format-specific
head and tail bytes (JPEG SOI/EOI, PNG signature/IEND, GIF header/3B,
WEBP RIFF size). Catches the production failure mode where a file
landed missing its 2-byte JPEG EOI marker and PIL.ImageFile.load
raised "image file is truncated (6 bytes not processed)" downstream.
Reads only 16 bytes from each end — O(1) per file. Unknown extensions
(JSON sidecars, ugoira zips, etc.) pass through. Hooks into the
download flow in a follow-up commit.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
