0.87.10 followup from the rereview. The fix lacked a test for the
specific axis it changed (admin authority via cookie only). New test
mints a bearer for a promoted admin and asserts PATCH/PUT-cover/DELETE-
cover all 403 on NULL-uploader rows. Mutation-confirmed.
Also: `MultipartBuilder::finalize` now `pub` for bearer-multipart use;
`admin_csrf_guard` rustdoc updated to describe the post-0.87.10
cookie-precedence rule.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>