Document the paired-word sharding certificate seqlock layout #1172

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: The slot allocations are corrected; documenting paired Int32 words is an additional design-note nit.

Owning path: replication/DESIGN.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: 6023c031ab85.

Review-Deferral: f24c0b24ca682444 severity=nit category=documentation

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Additional review identifiers for this root cause

Review-Deferral: 3fa4dcbdd0a709e8 severity=nit category=documentation