feat: Session（Segment 群の grouping）を導入

- SessionId 型を新設、各 SegmentStart に session_id を持たせる
- compaction / 内部 fork は同 SessionId を継承、fork() は新 Session を発行
- Store API を (SessionId, SegmentId) ベースに、FsStore layout は
  <root>/<session_id>/<segment_id>.jsonl に
- Store::list_sessions / list_segments(session_id) / lookup_session_of を追加
- restore_by_segment shim を session-store に提供（pod-cli --session で使用）
- SegmentState に SegmentLocation (session_id, segment_id) を保持し ArcSwap で更新
- RestoredState に session_id: Option<SessionId> を追加
- Picker は Session 単位に列挙、leaf segment を解決して resume

crates/session-store/src/lib.rs

@@ -1,10 +1,14 @@
-//! Segment persistence via append-only JSONL logs.
+//! Session persistence via append-only JSONL logs.
 //!
 //! # Architecture
 //!
-//! Sessions are recorded as a sequence of [`LogEntry`] values, one per line
-//! in a `.jsonl` file. Reading the log and collecting entries reconstructs
-//! the full Worker state — no separate snapshots or checkpoints needed.
+//! A [`Session`](SessionId) is a fork-tree of [`Segment`](SegmentId)s
+//! belonging to the same logical conversation. Each Segment is recorded
+//! as a sequence of [`LogEntry`] values, one per line in a `.jsonl`
+//! file. Reading a segment log and collecting entries reconstructs the
+//! Worker state at that segment — no separate snapshots or checkpoints
+//! needed. Compaction and fork operations mint a fresh Segment within
+//! the same Session.
 //!
 //! This crate provides free functions for persistence operations.
 //! The caller (typically Pod) holds the Worker directly and calls these
@@ -19,7 +23,7 @@
 //! use session_store::{create_segment, restore, save_delta, FsStore, SegmentStartState};
 //!
 //! let store = FsStore::new("./sessions")?;
-//! let segment_id = create_segment(&store, SegmentStartState {
+//! let (session_id, segment_id) = create_segment(&store, SegmentStartState {
 //!     system_prompt: None,
 //!     config: &config,
 //!     history: &[],
@@ -41,9 +45,10 @@ pub use llm_worker::llm_client::types::{ContentPart, Item, Role};
 pub use logged_item::{LoggedContentPart, LoggedItem, LoggedRole, from_logged, to_logged};
 pub use segment::{
     SegmentStartState, append_entry, append_system_item, classify_history_item,
-    create_compacted_segment, create_segment, create_segment_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,
+    create_compacted_segment, create_segment, create_segment_with_ids, ensure_head_or_fork, fork,
+    fork_at, restore, restore_by_segment, 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 segment_log::{
     LogEntry, POD_SCOPE_EXTENSION_DOMAIN, PodScopeSnapshot, RestoredState, SegmentOrigin,
@@ -52,9 +57,21 @@ pub use segment_log::{
 pub use system_item::{SystemItem, render_pod_event};
 pub use store::{Store, StoreError};
 
+/// Session identifier — the fork-tree root. UUID v7 (time-ordered).
+///
+/// All Segments belonging to the same Session share this ID. Compaction
+/// and fork operations create a new Segment within the same Session, so
+/// `WHERE session_id = ?` retrieves the full lineage.
+pub type SessionId = uuid::Uuid;
+
 /// Segment identifier. UUID v7 (time-ordered, lexicographically sortable).
 pub type SegmentId = uuid::Uuid;
 
+/// Generate a new session ID.
+pub fn new_session_id() -> SessionId {
+    uuid::Uuid::now_v7()
+}
+
 /// Generate a new segment ID.
 pub fn new_segment_id() -> SegmentId {
     uuid::Uuid::now_v7()