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/protocol/src/protocol.rs

@@ -3334,6 +3334,8 @@ impl Mul<f64> for TruncationPolicy {
 #[derive(Serialize, Deserialize, Clone, JsonSchema)]
 pub struct RolloutLine {
     pub timestamp: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub ordinal: Option<u64>,
     #[serde(flatten)]
     pub item: RolloutItem,
 }