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-metrics/src/lib.rs

@@ -18,7 +18,7 @@
 use std::collections::BTreeMap;
 
 use serde::{Deserialize, Serialize};
-use session_store::{EntryHash, SessionId, Store, StoreError, save_extension, session_log};
+use session_store::{SessionId, Store, StoreError, save_extension, session_log};
 
 /// Domain tag used in `LogEntry::Extension` for all metrics records.
 pub const DOMAIN: &str = "metrics";
@@ -78,11 +78,10 @@ impl Metric {
 pub fn record_metric(
     store: &impl Store,
     session_id: SessionId,
-    head_hash: &mut Option<EntryHash>,
     metric: &Metric,
 ) -> Result<(), StoreError> {
     let payload = serde_json::to_value(metric).expect("Metric serialization cannot fail");
-    save_extension(store, session_id, head_hash, DOMAIN, payload)
+    save_extension(store, session_id, DOMAIN, payload)
 }
 
 /// `RestoredState.extensions` から metrics domain の payload を順に取り出し、