fix(session): finish interrupted deletion without removing shared recovery [skip ci]

Allow retry after a deletion tombstone while retaining the legacy single-slot checkpoint, which older binaries can write outside the per-session lock. The existing lifecycle case now checks its bytes remain intact and the retired checkpoint stays hidden.

Refs #7095 (CW-7553). On frozen 896e781fbb plus 29 inventoried dirty paths, 33 existing checkpoint, deletion, retention and late-usage cases passed; rustfmt and diff check passed. This does not reproduce the original Deepin report or qualify older concurrent binaries.

crates/tui/src/session_manager.rs

@@ -4355,7 +4355,8 @@ impl SessionManager {
         }
         let owns_legacy_checkpoint =
             matches!(&legacy_origin, Ok(Some(origin)) if origin == id.trim());
-        if matches!(removal, SessionRemoval::Explicit) && owns_legacy_checkpoint {
+        if matches!(removal, SessionRemoval::Explicit) && owns_legacy_checkpoint && !already_deleted
+        {
             return Err(io::Error::other(
                 "this session has a shared legacy checkpoint; remove that checkpoint separately before deleting the session",
             ));

crates/tui/src/session_manager/test_cases_01.rs

@@ -859,6 +859,8 @@
                 .as_bytes(),
         )
         .expect("legacy checkpoint before deletion");
+        let legacy = manager.checkpoints_dir().join(LEGACY_CHECKPOINT_FILE);
+        let legacy_before = fs::read(&legacy).expect("legacy bytes");
         let (ledger, _) = manager
             .ensure_late_usage_paths("interrupted-delete")
             .expect("paths");
@@ -891,7 +893,8 @@
             .delete_session("interrupted-delete")
             .expect("finish deletion");
         assert!(manager.list_sessions().expect("list").is_empty());
-        assert!(manager.list_checkpoints().expect("checkpoints").is_empty());
+        assert_eq!(fs::read(&legacy).expect("legacy preserved"), legacy_before);
+        assert!(manager.load_legacy_checkpoint().expect("legacy read").is_none());
     }
 
     #[test]