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/examples/pod_cli.rs

@@ -54,7 +54,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut pod = Pod::from_manifest_toml(&toml, store).await?;
     let manifest: &PodManifest = pod.manifest();
     println!("Pod: {}", manifest.pod.name);
-    println!("Session: {}", pod.session_id());
+    println!("Session: {}", pod.segment_id());
 
     // 4. Run a prompt
     let result = pod.run_text("What is the capital of France?").await?;
@@ -76,7 +76,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     }
 
     // 6. Session ID for potential restore
-    println!("\nSession ID: {}", pod.session_id());
+    println!("\nSession ID: {}", pod.segment_id());
 
     Ok(())
 }