[rollout_trace] Record core session rollout traces (#18877)

## Summary

Wires rollout trace recording into `codex-core` session and turn
execution. This records the core model request/response, compaction, and
session lifecycle boundaries needed for replay without yet tracing every
nested runtime/tool boundary.

## Stack

This is PR 2/5 in the rollout trace stack.

- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command

## Review Notes

This layer is the first live integration point. The important review
question is whether trace recording is isolated from normal session
behavior: trace failures should not become user-visible execution
failures, and recording should preserve the existing turn/session
lifecycle semantics.

The PR depends on the reducer/data model from the first stack entry and
only introduces the core recorder surface that later PRs use for richer
runtime and relationship events.

codex-rs/core/src/session/mod.rs

@@ -120,6 +120,8 @@ use codex_protocol::request_user_input::RequestUserInputResponse;
 use codex_rmcp_client::ElicitationResponse;
 use codex_rollout::RolloutConfig;
 use codex_rollout::state_db;
+use codex_rollout_trace::RolloutTraceRecorder;
+use codex_rollout_trace::ThreadStartedTraceMetadata;
 use codex_sandboxing::policy_transforms::intersect_permission_profiles;
 use codex_shell_command::parse_command::parse_command;
 use codex_terminal_detection::user_agent;
@@ -397,6 +399,8 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) metrics_service_name: Option<String>,
     pub(crate) inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
     pub(crate) inherited_exec_policy: Option<Arc<ExecPolicyManager>>,
+    /// Parent rollout-tree recorder, or a disabled recorder when this spawn has no parent trace.
+    pub(crate) inherited_rollout_trace: RolloutTraceRecorder,
     pub(crate) user_shell_override: Option<shell::Shell>,
     pub(crate) parent_trace: Option<W3cTraceContext>,
     pub(crate) analytics_events_client: Option<AnalyticsEventsClient>,
@@ -452,6 +456,7 @@ impl Codex {
             inherited_shell_snapshot,
             user_shell_override,
             inherited_exec_policy,
+            inherited_rollout_trace,
             parent_trace: _,
             analytics_events_client,
         } = args;
@@ -647,6 +652,7 @@ impl Codex {
             agent_control,
             environment_manager,
             analytics_events_client,
+            inherited_rollout_trace,
         )
         .await
         .map_err(|e| {