update: 残存 Session 識別子の Segment 化（review follow-up）

レビュー指摘の通り、次の session-grouping-introduce で新 SessionId が
入る前に名称衝突を避けるため取り残しを掃除。

- PodError::Session{Empty,ScopeMissing} → Segment{Empty,ScopeMissing}
- ScopeLockError::SessionConflict → SegmentConflict
- Pod.session_state / SegmentState.set_session_id 系
- source_session_id / prev_session_id / ensure_session_head / short_session
- pod_cli の "Session ID:" 表示
- fs_store の sessions ローカル変数

crates/session-store/src/segment.rs

@@ -60,7 +60,7 @@ pub fn create_segment_with_id(
 pub fn create_compacted_segment(
     store: &impl Store,
     state: SegmentStartState<'_>,
-    source_session_id: SegmentId,
+    source_segment_id: SegmentId,
     source_turn_count: usize,
 ) -> Result<SegmentId, StoreError> {
     let segment_id = crate::new_segment_id();
@@ -71,7 +71,7 @@ pub fn create_compacted_segment(
         history: to_logged(state.history),
         forked_from: None,
         compacted_from: Some(SegmentOrigin {
-            segment_id: source_session_id,
+            segment_id: source_segment_id,
             at_turn_index: source_turn_count,
         }),
     };