update: entry hash chain と session_head mutex を撤廃

- HashedEntry / EntryHash / compute_hash / build_chain 撤去、JSONL は 1 行 1 LogEntry
- SessionOrigin.at_hash → at_turn_index (TurnEnd 由来) に置換
- Pod 側 SessionHead mutex を ArcSwap<SessionId> + AtomicUsize の SessionState に置換
- ensure_head_or_fork は store の entry count と writer の append tally で判定
- session-store から sha2 / hex 依存、pod から parking_lot 依存を削除

crates/session-store/src/lib.rs

@@ -40,15 +40,14 @@ pub use llm_worker::UsageRecord;
 pub use llm_worker::llm_client::types::{ContentPart, Item, Role};
 pub use logged_item::{LoggedContentPart, LoggedItem, LoggedRole, from_logged, to_logged};
 pub use session::{
-    SessionStartState, append_entry, append_entry_with_hash, append_system_item,
-    classify_history_item, create_compacted_session, create_session, create_session_with_id,
-    ensure_head_or_fork, fork, fork_at, restore, save_config_changed, save_delta, save_extension,
-    save_pod_scope, save_run_completed, save_run_errored, save_turn_end, save_usage,
-    save_user_input,
+    SessionStartState, append_entry, append_system_item, classify_history_item,
+    create_compacted_session, create_session, create_session_with_id, ensure_head_or_fork, fork,
+    fork_at, restore, save_config_changed, save_delta, save_extension, save_pod_scope,
+    save_run_completed, save_run_errored, save_turn_end, save_usage, save_user_input,
 };
 pub use session_log::{
-    EntryHash, HashedEntry, LogEntry, POD_SCOPE_EXTENSION_DOMAIN, PodScopeSnapshot, RestoredState,
-    SessionOrigin, build_chain, collect_state, compute_hash,
+    LogEntry, POD_SCOPE_EXTENSION_DOMAIN, PodScopeSnapshot, RestoredState, SessionOrigin,
+    collect_state,
 };
 pub use system_item::{SystemItem, render_pod_event};
 pub use store::{Store, StoreError};