Add ordinals to paginated rollout records (#32332)

## Why

Paginated thread history needs durable ordering so consumers can process a rollout suffix without rebuilding all earlier history.

## What changed

- Add optional, zero-based ordinals to `RolloutLine` records in paginated rollouts while leaving legacy rollout serialization unchanged.
- Continue ordinals from the last valid record when appending or resuming, including after gaps or an incomplete tail, and reject overflow without appending.
- Add a stateless `project_rollout_line` helper that maps canonical turn lifecycle and completed-item records into thread-history change sets.

## Testing

- Cover ordinal assignment, legacy compatibility, resume and tail recovery, overflow handling, and thread-history projection for completed, failed, and interrupted turns.

GitOrigin-RevId: 3a9bb6cd2a1f674a9f154342e96e9aa3d8330781

codex-rs/rollout/src/compression_tests.rs

@@ -483,10 +483,12 @@ fn write_rollout(path: &std::path::Path, thread_id: ThreadId, message: &str) ->
     let lines = [
         RolloutLine {
             timestamp: "2025-01-03T12:00:00Z".to_string(),
+            ordinal: None,
             item: RolloutItem::SessionMeta(session_meta_line),
         },
         RolloutLine {
             timestamp: "2025-01-03T12:00:01Z".to_string(),
+            ordinal: None,
             item: RolloutItem::EventMsg(EventMsg::UserMessage(UserMessageEvent {
                 message: message.to_string(),
                 ..Default::default()