Base copy records COPY_START[3] relayed anchors as receiver cursors at COPY_COMPLETE before the copy tail drains #1056

Summary

When a fresh base copy starts (replication/replicationConnection.ts), the sender collects each relayed origin log's last committed key (collectRelayedLogAnchors) before the table walk and sends that map in COPY_START[3]. On the receiver, maybeFinishCopy (replication/replicationConnection.ts:4026-4033) calls noteOriginProgress(originId, key) for every entry in copyOriginAnchors as soon as COPY_COMPLETE arrives and in-flight copy blobs settle — before the sender's append-order copy tail (boundaryRange) has streamed entries appended to those relayed logs during the copy walk.

Impact

For an exact-anchored relayed log, the table walk covers all records present in the primary tables when the walk scanned them, while the subsequent append-order copy tail (resumeAfterExactStart) is responsible for delivering any relayed transaction appended to the copy source's log after anchor was snapshotted — including transactions whose origin timestamp key is older than anchor (late-arriving relayed writes).

Because the receiver persists anchor as its originLogKey cursor at COPY_COMPLETE:

1. If the connection drops after COPY_COMPLETE finishes and before the copy tail delivers a post-anchor relayed entry whose key is more than ORIGIN_CURSOR_OVERLAP_MS (60 s) below anchor,
2. The subsequent reconnect sends anchor in SUBSCRIPTION_REQUEST[4] and the sender resumes that origin log by key at anchor - 60 s rather than in append order past anchor, skipping that late-committed entry.

This is the same 60 s overlap heuristic that any reconnect after the tail already relies on (replication/DESIGN.md items 7 and 26, tracked toward closed-floor resume in #922), but recording COPY_START[3] at COPY_COMPLETE opens that reconnect window before the initial copy tail has drained.

Possible direction

Wait to seed the receiver's per-origin cursors from COPY_START[3] until the first post-copy end_txn trailer confirms the initial copy tail pass has drained (or subsume under durable append-order / closed-floor resume in #922).

Review-Deferral: 3a4a5d8c28e26a16 severity=nit category=data-integrity