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/usage.rs

@@ -64,7 +64,7 @@ impl UsageRecordSnapshot {
 pub struct UsageEvent {
     pub id: Uuid,
     pub occurred_at: DateTime<Utc>,
-    pub session_id: String,
+    pub segment_id: String,
     pub event: UsageEventKind,
     pub source: UsageSource,
     pub records: Vec<UsageRecordSnapshot>,
@@ -72,7 +72,7 @@ pub struct UsageEvent {
 
 impl UsageEvent {
     pub fn new(
-        session_id: impl Into<String>,
+        segment_id: impl Into<String>,
         event: UsageEventKind,
         source: UsageSource,
         records: Vec<UsageRecordSnapshot>,
@@ -80,7 +80,7 @@ impl UsageEvent {
         Self {
             id: Uuid::now_v7(),
             occurred_at: Utc::now(),
-            session_id: session_id.into(),
+            segment_id: segment_id.into(),
             event,
             source,
             records,
@@ -144,7 +144,7 @@ pub fn append_usage_event(layout: &WorkspaceLayout, event: &UsageEvent) -> io::R
 /// Convenience for a successful explicit record read.
 pub fn append_use_event(
     layout: &WorkspaceLayout,
-    session_id: impl Into<String>,
+    segment_id: impl Into<String>,
     source: UsageSource,
     records: Vec<UsageRecordSnapshot>,
 ) -> io::Result<()> {
@@ -153,14 +153,14 @@ pub fn append_use_event(
     }
     append_usage_event(
         layout,
-        &UsageEvent::new(session_id, UsageEventKind::Use, source, records),
+        &UsageEvent::new(segment_id, UsageEventKind::Use, source, records),
     )
 }
 
 /// Convenience for resident model-invocation exposure cost telemetry.
 pub fn append_resident_exposure_event(
     layout: &WorkspaceLayout,
-    session_id: impl Into<String>,
+    segment_id: impl Into<String>,
     records: Vec<UsageRecordSnapshot>,
 ) -> io::Result<()> {
     if records.is_empty() {
@@ -169,7 +169,7 @@ pub fn append_resident_exposure_event(
     append_usage_event(
         layout,
         &UsageEvent::new(
-            session_id,
+            segment_id,
             UsageEventKind::ResidentExposure,
             UsageSource::ResidentInjection,
             records,