core: document compaction tail merge race

Explain the snapshot-to-replace race at both compaction call sites so the extra re-snapshot and append-only tail preservation logic is easier to review.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/compact.rs

@@ -211,6 +211,9 @@ async fn run_compact_task_inner(
         .cloned()
         .collect();
     new_history.extend(ghost_snapshots);
+    // Compaction snapshots history, waits on a model call, then replaces
+    // session history wholesale. Background writers can append during that
+    // window, so re-snapshot here and preserve any append-only tail items.
     let latest_history_snapshot = sess.clone_history().await;
     if !append_concurrent_history_tail_if_append_only(
         &mut new_history,

codex-rs/core/src/compact_remote.rs

@@ -151,6 +151,9 @@ async fn run_remote_compact_task_inner_impl(
     if !ghost_snapshots.is_empty() {
         new_history.extend(ghost_snapshots);
     }
+    // Remote compaction snapshots history, waits on an API call, then replaces
+    // session history wholesale. Background writers can append during that
+    // window, so re-snapshot here and preserve any append-only tail items.
     let latest_history_snapshot = sess.clone_history().await;
     if !append_concurrent_history_tail_if_append_only(
         &mut new_history,