The remaining reseed defect cannot be diagnosed from the current status surface.
A stream key is a per-LEADER-REGION id (`shard_of_region`), not a shard group, so
a group accumulates one key per leadership it has followed — but every status field
reports only the CURRENT leader's key. A position retained from a previous
leadership is therefore invisible, while the receiver's gap check
(`receiver.rs`: `request_catchup(key, applied + 1)`) will chase ANY key that
received data this round.
That is the blind spot: live tidaldb-0 pulls `from_seqno=13540653`, so some key
sits at 13540652, while the group it reports on converged at 13540661 — and
nothing in the status can say which key that is.
Adds `ReplicationState::applied_by_key` and surfaces it as `applied_by_key` on the
status response. Instrument only: no behavioural change. Same instrument-first move
that turned the previous two defects into one-run diagnoses instead of speculation.
925a616cda