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/tests.rs

@@ -1314,6 +1314,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
     let conversation_id = ThreadId::from_string(&uuid.to_string())?;
     let meta_line = RolloutLine {
         timestamp: ts.to_string(),
+        ordinal: None,
         item: RolloutItem::SessionMeta(SessionMetaLine {
             meta: SessionMeta {
                 session_id: conversation_id.into(),
@@ -1345,6 +1346,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
 
     let user_event_line = RolloutLine {
         timestamp: ts.to_string(),
+        ordinal: None,
         item: RolloutItem::EventMsg(EventMsg::UserMessage(UserMessageEvent {
             client_id: None,
             message: "hello".into(),
@@ -1360,6 +1362,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
     for idx in 0..total_messages {
         let response_line = RolloutLine {
             timestamp: format!("{ts}-{idx:02}"),
+            ordinal: None,
             item: RolloutItem::ResponseItem(ResponseItem::Message {
                 id: None,
                 role: "assistant".into(),