feat: include submitted_at in review proposal identity

[fentonbenjamin]

Adds a stable hashed identity for review proposals so we can dedupe at the queue layer. Includes submitted_at in the canonical payload so we can sort + group by submission time later. New lib/proposal-id.ts.

_{scrimmage scenario: idempotency-timestamp · tag: 7adf1e}

[not-a-magic]

Not Magic — advisory

Record scratch: lib/proposal-id.ts: large addition (19 lines added)

Source:
lib/proposal-id.ts

To restore reliance:
review the change.

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_{Show what this relies on · Chain #133 · Advisory only.}