Materialize rollout state on-demand for v2 thread APIs

### Change summary

Defer rollout file creation until needed.
* Add a core API to force rollout persistence for loaded non-ephemeral threads:
* seeds initial context if needed
* flushes rollout and returns persisted path

Add concurrency guard to make lazy rollout initialization idempotent under concurrent calls.

Add centralized app-server rollout-path resolver that:
* uses in-memory thread state when loaded
* forces persistence on demand for rollout-dependent calls
* falls back to on-disk lookup for unloaded threads
* maps ephemeral threads to invalid-request errors for rollout-dependent operations

Route rollout-dependent endpoints through the resolver (v2 + shared legacy surfaces), including:
* thread/archive
* thread/resume (thread-id path)
* thread/fork (thread-id path)
* resumeConversation
* forkConversation
* thread summary by thread id
* detached review parent-thread path resolution
* feedback include_logs rollout resolution

Remove stale cached rollout-path assumptions in rollback/detached-review flows by resolving via thread id when needed.

No wire-schema changes; behavior-only change.

v1 compatibility is not expanded in this PR.

### Tests updated/added

* thread_start: assert rollout is absent immediately after thread/start; created after first completed turn.
* thread_resume: resume by thread id succeeds for just-started thread via on-demand persistence; path-vs-thread-id precedence test updated.
* thread_fork: fork by thread id succeeds for just-started thread.
* thread_archive: archive succeeds for just-started thread and materializes before archive.
* thread_unarchive: adjusted for deferred creation timing.
* thread_rollback: rollback path no longer depends on stale cached rollout path.
* Detached review targeted test verified for lazy path behavior.
* Core tests for new persistence API

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -10,6 +10,8 @@ use codex_app_server_protocol::SessionSource;
 use codex_app_server_protocol::ThreadForkParams;
 use codex_app_server_protocol::ThreadForkResponse;
 use codex_app_server_protocol::ThreadItem;
+use codex_app_server_protocol::ThreadStartParams;
+use codex_app_server_protocol::ThreadStartResponse;
 use codex_app_server_protocol::ThreadStartedNotification;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
@@ -117,6 +119,50 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn thread_fork_materializes_rollout_for_fresh_thread() -> Result<()> {
+    let server = create_mock_responses_server_repeating_assistant("Done").await;
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let start_id = mcp
+        .send_thread_start_request(ThreadStartParams::default())
+        .await?;
+    let start_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(start_id)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(start_resp)?;
+
+    let fork_id = mcp
+        .send_thread_fork_request(ThreadForkParams {
+            thread_id: thread.id.clone(),
+            ..Default::default()
+        })
+        .await?;
+    let fork_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(fork_id)),
+    )
+    .await??;
+    let ThreadForkResponse {
+        thread: forked_thread,
+        ..
+    } = to_response::<ThreadForkResponse>(fork_resp)?;
+
+    assert_ne!(forked_thread.id, thread.id);
+    assert!(
+        forked_thread.path.is_some(),
+        "forked thread should have a persisted rollout path"
+    );
+
+    Ok(())
+}
+
 // Helper to create a config.toml pointing at the mock model server.
 fn create_config_toml(codex_home: &Path, server_uri: &str) -> std::io::Result<()> {
     let config_toml = codex_home.join("config.toml");