update: SessionId / SessionStart / SessionOrigin 等を Segment 系名称へ

- Type/Function/Variantを Segment* 系へ統一
  - SessionId/SessionStart/SessionOrigin/SessionStartState/SessionState/SessionLogSink/SessionLockInfo
  - new_session_id / session_id / create_session* / list_sessions / lookup_session / update_session / find_by_session
  - protocol Event::SessionRotated → SegmentRotated、CompactDone.new_session_id → new_segment_id
- Module: session_log → segment_log / session → segment (file mv 含む)
  pod 側の session_log_sink → segment_log_sink も同様
- crate 名 (session-store)、CLI flag (--session)、ResumeWithSession (CLI tied) は据え置き
- session-tests/session_metrics_test 等の Store impl も追従

crates/pod/src/runtime/dir.rs

@@ -131,7 +131,7 @@ mod tests {
     fn test_state() -> PodSharedState {
         PodSharedState::new(
             "test-pod".into(),
-            session_store::new_session_id(),
+            session_store::new_segment_id(),
             "[pod]\nname = \"test-pod\"".into(),
             protocol::Greeting {
                 pod_name: "test-pod".into(),