fix(ingest): keep three request costs bounded per request - #2566

🟢 Confidence 10/10 · safe to merge. The replay decode now reserves from decode_memory before inflating and the shed/bounds paths are each pinned by a new test.
_{no findings · tests covered · risk medium · 2/2 new units observable}

Replay chunk inflate moves to the blocking pool inside the shared decode-memory budget, Vercel audit deliveries are capped at 10,000 events, and ingest credentials over 256 bytes are ignored. The earlier budget-escape finding is fixed; nothing else in the diff is unsafe.

• Replay inflate runs on the blocking pool inside state.decode_memory
• gzip_declared_len stops the inflate at the size reserved
• Audit deliveries past AUDIT_TRAIL_MAX_EVENTS answer 413
• extract_ingest_key ignores values longer than 256 bytes

Fixed since the last review

• ✅ F1 · Replay decode on the blocking pool escapes state.decode_memory

_{58bcbb1 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}