In-process, thread-safe last-request-summary sink (per account_id) with a
public last_request_summary(account_id:) reader so FilteredCountInstrumentation's
documented local observability signals are consumable instead of discarded.
No APM/New Relic or any off-box egress; sink and reader stay fully in-process.
Adds regression specs asserting the completed summary (status, counts,
duration) is readable and an unknown account returns nil.
2ef6fa554b