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-registry/src/test_util.rs

@@ -6,12 +6,12 @@ use std::path::{Path, PathBuf};
 use std::sync::{LazyLock, Mutex, MutexGuard};
 
 use manifest::{Permission, ScopeRule};
-use session_store::SessionId;
+use session_store::SegmentId;
 
 use crate::table::LockFileGuard;
 
-pub(crate) fn sid() -> SessionId {
-    session_store::new_session_id()
+pub(crate) fn sid() -> SegmentId {
+    session_store::new_segment_id()
 }
 
 /// Serialises tests that mutate runtime-dir env vars. The test