feat(rollout): persist canonical items for paginated threads

[owenlin0]

Description

This PR is the final CA-629 persistence layer on top of #30283.

Paginated rollouts now persist completed canonical TurnItem snapshots instead of the parallel legacy item-shaped events. Legacy rollouts keep the existing persistence behavior.

Why

The later SQLite projector needs to read the JSONL suffix and materialize paginated ThreadItems without reconstructing the whole thread first. That only works if new paginated rollouts already contain stable turn/item IDs and completed item snapshots.

I left legacy rollout persistence alone because old binaries and historical rollouts still understand the old event shapes. The new storage contract is selected per thread with historyMode = "paginated".

What changed

• Made rollout persistence history-mode aware: paginated threads keep completed canonical ItemCompleted snapshots and drop redundant persisted legacy item-shaped events; legacy threads keep current behavior.
• Moved rollout canonicalization into LiveThread, so ThreadStore::append_items receives the already-filtered durable batch.
• Made paginated threads force stable response item IDs even when Feature::ItemIds is off.
• Added persisted turn/item IDs for review enter/exit events so replay can use the same IDs the live app-server path saw.
• Updated metadata/list/search paths to understand completed canonical user-message items.
• Switched synthetic replay/backfill turn/item IDs to globally unique values needed by the later SQLite schema.

Validation

• cargo check -p codex-protocol -p codex-app-server-protocol -p codex-rollout -p codex-thread-store -p codex-core -p codex-app-server -p codex-state

I did not run the full codex-core suite.

Follow-up

The later SQLite projection work can now consume completed canonical item snapshots from paginated rollout JSONL without reconstructing the full thread first.