core: add context window lineage IDs (#29256)

## Why

The rendered `<token_budget>` fragment identifies the thread and current
context window, but it does not expose enough lineage to identify the
first window in the thread or the immediately preceding window. Those
IDs also need to remain stable across compaction, resume, and rollback.

## What changed

- Track first, previous, and current UUIDv7 context-window IDs in
auto-compaction state.
- Render `thread_id`, `first_window_id`, `previous_window_id`, and the
current window ID in the full `<token_budget>` fragment.
- Persist the first and previous window IDs in compacted rollout
checkpoints and restore them during rollout reconstruction.
- Preserve compatibility with older compacted records that do not
contain the new optional fields.
- Update focused state, rendering, reconstruction, rollback, and
serialization coverage.

## Validation

- `just test -p codex-core token_budget`
- `just test -p codex-protocol compacted_item::tests`
- `just test -p codex-core tracks_prefill_and_window_boundaries`
- `just test -p codex-core
reconstruct_history_uses_replacement_history_verbatim`
- `just test -p codex-core
thread_rollback_restores_cleared_reference_context_item_after_compaction`

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

@@ -311,6 +311,7 @@ use crate::session_startup_prewarm::SessionStartupPrewarmHandle;
 use crate::shell;
 #[cfg(test)]
 use crate::skills::SkillLoadOutcome;
+use crate::state::AutoCompactWindowIds;
 use crate::state::AutoCompactWindowSnapshot;
 use crate::state::PendingRequestPermissions;
 use crate::state::SessionServices;
@@ -1364,6 +1365,8 @@ impl Session {
             previous_turn_settings,
             reference_context_item,
             window_number,
+            first_window_id,
+            previous_window_id,
             window_id,
         } = self
             .reconstruct_history_from_rollout(turn_context, rollout_items)
@@ -1382,8 +1385,16 @@ impl Session {
         {
             let mut state = self.state.lock().await;
             state.replace_history(history, reference_context_item);
-            let window_id = window_id.unwrap_or_else(|| state.auto_compact_window_id());
-            state.restore_auto_compact_window(window_number, window_id);
+            let fallback_ids = state.auto_compact_window_ids();
+            let window_id = window_id.unwrap_or(fallback_ids.window_id);
+            state.restore_auto_compact_window(
+                window_number,
+                AutoCompactWindowIds {
+                    first_window_id: first_window_id.unwrap_or(window_id),
+                    previous_window_id,
+                    window_id,
+                },
+            );
             state.set_previous_turn_settings(previous_turn_settings.clone());
         }
         let prefix_tokens = if matches!(
@@ -3011,7 +3022,7 @@ impl Session {
             collaboration_mode,
             base_instructions,
             session_source,
-            auto_compact_window_id,
+            auto_compact_window_ids,
         ) = {
             let state = self.state.lock().await;
             (
@@ -3020,7 +3031,7 @@ impl Session {
                 state.session_configuration.collaboration_mode.clone(),
                 state.session_configuration.base_instructions.clone(),
                 state.session_configuration.session_source.clone(),
-                state.auto_compact_window_id(),
+                state.auto_compact_window_ids(),
             )
         };
         if let Some(model_switch_message) =
@@ -3210,7 +3221,9 @@ impl Session {
             developer_sections.push(
                 crate::context::TokenBudgetContext::new(
                     self.thread_id(),
-                    auto_compact_window_id,
+                    auto_compact_window_ids.first_window_id,
+                    auto_compact_window_ids.previous_window_id,
+                    auto_compact_window_ids.window_id,
                     model_context_window,
                 )
                 .render(),
@@ -3309,10 +3322,9 @@ impl Session {
         format!("{thread_id}:{window_number}")
     }
 
-    pub(crate) async fn advance_auto_compact_window(&self) -> (u64, String) {
+    pub(crate) async fn advance_auto_compact_window(&self) -> (u64, AutoCompactWindowIds) {
         let mut state = self.state.lock().await;
-        let (window_number, window_id) = state.advance_auto_compact_window();
-        (window_number, window_id.to_string())
+        state.advance_auto_compact_window()
     }
 
     pub(crate) async fn request_new_context_window(&self) {
@@ -3328,7 +3340,7 @@ impl Session {
             let mut state = self.state.lock().await;
             state.start_new_context_window_if_requested()
         };
-        let (window_number, window_id) = window?;
+        let (window_number, window_ids) = window?;
         let context_items = self.build_initial_context(turn_context).await;
         let turn_context_item = turn_context.to_turn_context_item();
         let replacement_history = context_items;
@@ -3341,7 +3353,9 @@ impl Session {
                 message: String::new(),
                 replacement_history: Some(replacement_history),
                 window_number: Some(window_number),
-                window_id: Some(window_id.to_string()),
+                first_window_id: Some(window_ids.first_window_id.to_string()),
+                previous_window_id: window_ids.previous_window_id.map(|id| id.to_string()),
+                window_id: Some(window_ids.window_id.to_string()),
             }),
             RolloutItem::TurnContext(turn_context_item),
         ])