Align the sharding guide route with its public terminology #768

Deferred from the closing review of #753 for declarative sharding (Pro #980). The owner explicitly scoped this root cause out of the closing implementation batch.

Current assessment: The existing residency guide route is valid; renaming it to match the public directive is editorial cleanup.

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

Follow-up: Implement and verify the scoped documentation or test improvement without expanding the current runtime contract.

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

Finding IDs: e2cd859b21ae, 68c9d5e35b37.

Review-Deferral: 4988dd48aee75b40 severity=nit category=documentation

Review-Deferral: 1dacd324d46b97cd severity=nit category=documentation

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