fix(ingest): a late audit event is still stored, and the Vercel drain stamps its own source - #2561

🟢 Confidence 10/10 · safe to merge. Two contained changes, each pinned by a test that fails before it; the shed exemption keys on a signal no sender can set.
_{no findings · tests covered · risk medium}

Exempts AuditTrail frames from the export worker's max-queue-age shed, and makes the Vercel audit drain stamp the integrated source instead of the key's default. Both changes are contained, off by default, and tested; safe to merge.

• export_and_mark no longer sheds frames whose signal is AuditTrail
• The Vercel audit drain stamps the verified claim's source via note_claimed_source_use
• Docs drop the claim that audit events appear on the Vercel overview

_{40d3dd7 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}