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/pod-registry/src/error.rs

@@ -30,7 +30,7 @@ pub enum ScopeLockError {
         "session {segment_id} is already held by pod `{pod_name}` at {}",
         .socket.display()
     )]
-    SessionConflict {
+    SegmentConflict {
         segment_id: SegmentId,
         pod_name: String,
         socket: PathBuf,

crates/pod-registry/src/lifecycle.rs

@@ -131,7 +131,7 @@ pub fn update_segment(pod_name: &str, new_segment_id: SegmentId) -> Result<(), S
     let mut guard = LockFileGuard::open(&lock_path)?;
     if let Some(other) = guard.data().find_by_segment(new_segment_id) {
         if other.pod_name != pod_name {
-            return Err(ScopeLockError::SessionConflict {
+            return Err(ScopeLockError::SegmentConflict {
                 segment_id: new_segment_id,
                 pod_name: other.pod_name.clone(),
                 socket: other.socket.clone(),
@@ -320,7 +320,7 @@ mod tests {
         // `a` cannot adopt b's live session id.
         let err = update_segment("a", s_b).unwrap_err();
         match err {
-            ScopeLockError::SessionConflict {
+            ScopeLockError::SegmentConflict {
                 pod_name,
                 segment_id,
                 ..
@@ -328,7 +328,7 @@ mod tests {
                 assert_eq!(pod_name, "b");
                 assert_eq!(segment_id, s_b);
             }
-            other => panic!("expected SessionConflict, got {other:?}"),
+            other => panic!("expected SegmentConflict, got {other:?}"),
         }
     }
 }

crates/pod-registry/src/mutate.rs

@@ -63,7 +63,7 @@ pub fn register_pod_with_deny(
         return Err(ScopeLockError::DuplicatePodName(pod_name));
     }
     if let Some(existing) = guard.data().find_by_segment(segment_id) {
-        return Err(ScopeLockError::SessionConflict {
+        return Err(ScopeLockError::SegmentConflict {
             segment_id,
             pod_name: existing.pod_name.clone(),
             socket: existing.socket.clone(),
@@ -588,7 +588,7 @@ mod tests {
         )
         .unwrap();
         // Second registration tries to grab the same segment_id under
-        // a different pod_name. Without the SessionConflict check both
+        // a different pod_name. Without the SegmentConflict check both
         // would succeed and race on the same jsonl.
         let err = register_pod(
             &mut g,
@@ -600,7 +600,7 @@ mod tests {
         )
         .unwrap_err();
         match err {
-            ScopeLockError::SessionConflict {
+            ScopeLockError::SegmentConflict {
                 segment_id,
                 pod_name,
                 ..
@@ -608,7 +608,7 @@ mod tests {
                 assert_eq!(segment_id, shared_session);
                 assert_eq!(pod_name, "first");
             }
-            other => panic!("expected SessionConflict, got {other:?}"),
+            other => panic!("expected SegmentConflict, got {other:?}"),
         }
     }
 }