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/memory/src/consolidate/lock.rs

@@ -260,7 +260,7 @@ mod tests {
         let (id_a, _) = write_staging(
             &layout,
             SourceRef {
-                session_id: "s".into(),
+                segment_id: "s".into(),
                 range: [0, 0],
             },
             ExtractedPayload::default(),
@@ -269,7 +269,7 @@ mod tests {
         let (id_b, _) = write_staging(
             &layout,
             SourceRef {
-                session_id: "s".into(),
+                segment_id: "s".into(),
                 range: [1, 1],
             },
             ExtractedPayload::default(),