[codex] Cover media MIME drift in merge validators

[adamziel]

What it does

Adds focused media-validator coverage for WordPress attachment post_mime_type drift. A staged .jpg upload declared as application/pdf now produces a plugin-scoped plugin-wp-media-mime-drift audit conflict.

Rationale

Uploads are a DB-plus-files semantic object. The existing validator checked attachment metadata, upload paths, generated sizes, file ownership, and filesize drift, but did not cover the attachment row's MIME metadata. A merge that leaves wp_posts.post_mime_type inconsistent with the uploaded file should be review-visible instead of silently accepted by the validator.

Implementation

tests/cow/media_validator.php now:

• includes post_mime_type in the attachment fixture schema,
• allows attachment fixtures to declare a MIME type,
• checks known image extensions (jpg, jpeg, png, gif, webp) against post_mime_type,
• records plugin-wp-media-mime-drift with declared and expected MIME evidence.

docs/merge-reliability.md now lists this as covered upload validator evidence.

Testing instructions

• php -l tests/cow/media_validator.php
• make test-cow-media-validator
• make test-cow-semantic-fast
• git diff --check origin/trunk...HEAD