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HEIF/AVIF store XMP as a meta-box `mime` item whose bytes live in mdat/idat, out of reach of the top-level uuid/jumb box stripper. An AI-label XMP packet there (TC260 AIGC, IPTC "Made with AI", IPTC 2025.1) was therefore left in place. isobmff.blank_ai_xmp_packets locates each XMP packet by its <?xpacket begin ... end?> delimiters and, if it carries an AI marker (_AI_LABEL_MARKERS), overwrites it with spaces of the SAME length. Equal length means no box size or iloc offset shifts -- the coded image stays bit-for-bit intact, the item stays structurally valid, only the AI label content is destroyed. Plain (non-AI) XMP is left alone, mirroring the top-level XMP-uuid content match. Wired into remove_ai_metadata's ISOBMFF branch after strip_c2pa_boxes. Chosen over exiftool (a non-bundled binary dep) to stay pure-Python and droplet-compatible; over full iinf/iloc surgery to avoid offset-rewrite corruption risk. The AI labels we target are all XMP, so this closes the practical gap. An Exif *item* inside the meta box (rare) still needs iinf/iloc surgery or exiftool -- documented. 4 new tests (TestMetaBoxXmpBlanking): AI packet blanked (same length, marker gone, surrounding image bytes intact), plain XMP preserved, no-packet no-op, and end-to-end remove_ai_metadata on a .heic. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>