Add first-class ingest idempotency / failure-classification proof so the ingestion category can move from A+ t

[sarthakagrawal927]

Work

2. Add first-class ingest idempotency / failure-classification proof so the
   ingestion category can move from A+ to S without relying on route presence.

Context

Migrated from STATUS.md on 2026-07-29 when Fleet made GitHub Issues the sole operational work queue.

This issue is the to-do. Link the implementing pull request with Closes #<issue>; the linked pull request makes the work in progress, and merge + issue closure makes it done.