update: 残存 Session 識別子の Segment 化（review follow-up）

レビュー指摘の通り、次の session-grouping-introduce で新 SessionId が
入る前に名称衝突を避けるため取り残しを掃除。

- PodError::Session{Empty,ScopeMissing} → Segment{Empty,ScopeMissing}
- ScopeLockError::SessionConflict → SegmentConflict
- Pod.session_state / SegmentState.set_session_id 系
- source_session_id / prev_session_id / ensure_session_head / short_session
- pod_cli の "Session ID:" 表示
- fs_store の sessions ローカル変数

crates/tui/src/picker.rs

@@ -72,7 +72,7 @@ struct Row {
     preview: String,
     /// `Some(pod_name)` when a live Pod currently holds an allocation
     /// for this session in `pods.json`. Picking such a row launches
-    /// `pod --session <UUID>` which will fail with `SessionConflict` —
+    /// `pod --session <UUID>` which will fail with `SegmentConflict` —
     /// the badge warns the user up-front.
     live_pod: Option<String>,
 }
@@ -300,7 +300,7 @@ fn row_line(row: &Row, selected: bool) -> Line<'_> {
     };
     let mut spans = vec![
         Span::raw(marker),
-        Span::styled(short_session(row.id), id_style),
+        Span::styled(short_segment(row.id), id_style),
         Span::raw("  "),
     ];
     if let Some(ref pod_name) = row.live_pod {
@@ -313,7 +313,7 @@ fn row_line(row: &Row, selected: bool) -> Line<'_> {
     Line::from(spans)
 }
 
-fn short_session(id: SegmentId) -> String {
+fn short_segment(id: SegmentId) -> String {
     let s = id.to_string();
     s.chars().take(8).collect()
 }

crates/tui/src/spawn.rs

@@ -445,7 +445,7 @@ fn draw_form(f: &mut Frame<'_>, form: &Form) {
     .split(area);
 
     let title_text = match form.resume_from {
-        Some(id) => format!("resume pod   session: {}", short_session(id)),
+        Some(id) => format!("resume pod   session: {}", short_segment(id)),
         None => "spawn pod".to_string(),
     };
     let title = Paragraph::new(Line::from(vec![Span::styled(
@@ -473,7 +473,7 @@ fn draw_form(f: &mut Frame<'_>, form: &Form) {
 
 /// First 8 hex digits of a UUID — short enough to skim, long enough
 /// to disambiguate inside a 10-row picker.
-pub(crate) fn short_session(id: SegmentId) -> String {
+pub(crate) fn short_segment(id: SegmentId) -> String {
     let s = id.to_string();
     s.chars().take(8).collect()
 }