Preserve conversation order across host clock corrections (#9772)

[yashranaway]

I am working on the clock-rollback bug in #9772 and would welcome feedback on the ordering approach before the PR is finalized.

Persisted orchestration event sequences already retain the correct order when the host clock moves backward, but projection queries, pagination, and client timelines still sort by wall time.

Proposed fix:

• Carry an immutable creation sequence through message, plan, activity, and turn projections, keeping original timestamps for display.
• Anchor a turn to its initiating user message so paginated history includes the prompt with its response.
• Use sequence-based keyset pagination with compatibility for existing cursors and older server snapshots.
• Backfill projection ordering from retained events without rewriting the event log, preserving imported history and recreated threads.
• Cover clock rollback, streaming, restart/reload, pagination, replay, and terminal-state selection with focused tests on server and both clients.

This is limited to conversation ordering and the related current-turn state. It does not add a new clock service or change provider protocols.

Model: GPT-6 Astra
Harness: T3 code