Detect stale same-name backups before admitting a sharded origin #1175

Deferred from the closing review of #1116 for declarative sharding (Pro #980). The owner explicitly scoped this root cause out of the closing implementation batch.

Current assessment: Arbitrarily stale same-name backups are excluded by the approved complete-original-name-backup requirement. Automatic detection is a recovery extension.

Owning path: replication/shardingBootstrap.ts. Review lines move as the companion branches integrate main; use the linked PR's current source.

Follow-up: Establish the boundary with a focused regression or code trace, then propose the smallest change consistent with the stated recovery and compatibility contracts.

This issue does not reopen #980's approved primary-key-only scope, complete-history requirement, or ban on write serialization. It does not block the current drafts. No release or backport is committed; milestone is intentionally unset. Found through source review and synthetic tests; there is no customer-specific incident evidence.

Triage: P3; effort High; type Feature. Low-priority follow-up: no demonstrated correctness failure or new release requirement. This is deliberately below the parent reliability priority.

Finding IDs: 389141547e45.

Review-Deferral: 3f9821d561262f6a severity=minor category=correctness

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Additional review identifiers for this root cause

Review-Deferral: 6eaf52f5dec550bb severity=minor category=data-integrity