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/Cargo.toml

@@ -11,8 +11,6 @@ serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 uuid = { workspace = true, features = ["v7", "serde"] }
 thiserror = { workspace = true }
-sha2 = { workspace = true }
-hex = "0.4.3"
 protocol = { workspace = true }
 tracing.workspace = true