Track inherited paginated rollout prefixes (#33930)

## What changed

- Add `HistoryPosition` to identify a source thread and an exclusive rollout
  prefix by ordinal and JSONL byte offset.
- Add optional `history_base` metadata to `SessionMeta` so a thread can record
  an inherited prefix of another paginated rollout.
- Default missing `history_base` metadata to `None` and omit it when unset to
  preserve compatibility with existing rollout metadata.

## Testing

- Verify that legacy session metadata without `history_base` deserializes with
  no inherited history position.

GitOrigin-RevId: 3b2b07a2dfee793d3b37b0a60529f856f167dbc1

codex-rs/rollout/src/compression_tests.rs

@@ -606,6 +606,7 @@ fn write_rollout(path: &std::path::Path, thread_id: ThreadId, message: &str) ->
             selected_capability_roots: Vec::new(),
             memory_mode: None,
             history_mode: Default::default(),
+            history_base: None,
             subagent_history_start_ordinal: None,
             multi_agent_version: None,
             context_window: None,

codex-rs/rollout/src/metadata_tests.rs

@@ -53,6 +53,7 @@ async fn extract_metadata_from_rollout_uses_session_meta() {
         selected_capability_roots: Vec::new(),
         memory_mode: None,
         history_mode: ThreadHistoryMode::Paginated,
+        history_base: None,
         subagent_history_start_ordinal: None,
         multi_agent_version: None,
         context_window: None,
@@ -150,6 +151,7 @@ async fn extract_metadata_from_rollout_returns_latest_memory_mode() {
         selected_capability_roots: Vec::new(),
         memory_mode: None,
         history_mode: Default::default(),
+        history_base: None,
         subagent_history_start_ordinal: None,
         multi_agent_version: None,
         context_window: None,
@@ -424,6 +426,7 @@ fn write_rollout_in_sessions_with_cwd(
         selected_capability_roots: Vec::new(),
         memory_mode: None,
         history_mode: Default::default(),
+        history_base: None,
         subagent_history_start_ordinal: None,
         multi_agent_version: None,
         context_window: None,

codex-rs/rollout/src/recorder.rs

@@ -831,6 +831,7 @@ impl RolloutRecorder {
                     selected_capability_roots,
                     memory_mode: (!config.generate_memories()).then_some("disabled".to_string()),
                     history_mode,
+                    history_base: None,
                     subagent_history_start_ordinal,
                     multi_agent_version,
                     context_window: initial_window_id.map(SessionContextWindow::new),

codex-rs/rollout/src/recorder_tests.rs

@@ -178,6 +178,7 @@ async fn state_db_init_backfills_before_returning() -> anyhow::Result<()> {
             selected_capability_roots: Vec::new(),
             memory_mode: None,
             history_mode: Default::default(),
+            history_base: None,
             subagent_history_start_ordinal: None,
             multi_agent_version: None,
             context_window: None,

codex-rs/rollout/src/session_index_tests.rs

@@ -45,6 +45,7 @@ fn write_rollout_with_metadata(path: &Path, thread_id: ThreadId) -> std::io::Res
                 selected_capability_roots: Vec::new(),
                 memory_mode: None,
                 history_mode: Default::default(),
+                history_base: None,
                 subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,

codex-rs/rollout/src/state_db_tests.rs

@@ -178,6 +178,7 @@ fn write_rollout_with_user_message(
                     selected_capability_roots: Vec::new(),
                     memory_mode: None,
                     history_mode: Default::default(),
+                    history_base: None,
                     subagent_history_start_ordinal: None,
                     multi_agent_version: None,
                     context_window: None,

codex-rs/rollout/src/tests.rs

@@ -1336,6 +1336,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
                 selected_capability_roots: Vec::new(),
                 memory_mode: None,
                 history_mode: Default::default(),
+                history_base: None,
                 subagent_history_start_ordinal: None,
                 multi_agent_version: None,
                 context_window: None,