wip(k1): checkpoint connection recovery rewrite

Capture the current unreleased K1 connection, recovery, lifecycle, viewer, and test work as a single known-bad baseline for subsequent fixes.

src/k1link/artifacts.py

@@ -14,7 +14,12 @@ def utc_now_iso() -> str:
 
 
 def write_json_atomic(path: Path, payload: Any) -> None:
-    """Write JSON without exposing a partially written artifact."""
+    """Write JSON without exposing or acknowledging a partial commit.
+
+    Flushing the temporary file protects its contents, but a crash can still
+    lose the directory entry created by ``replace``. The parent directory is
+    therefore flushed after the atomic rename as the second durability edge.
+    """
     path = path.expanduser()
     path.parent.mkdir(parents=True, exist_ok=True)
     serialized = json.dumps(payload, ensure_ascii=False, indent=2) + "\n"
@@ -34,6 +39,13 @@ def write_json_atomic(path: Path, payload: Any) -> None:
             stream.flush()
             os.fsync(stream.fileno())
         Path(temp_name).replace(path)
+        directory_flags = os.O_RDONLY | getattr(os, "O_CLOEXEC", 0)
+        directory_flags |= getattr(os, "O_DIRECTORY", 0)
+        directory_descriptor = os.open(path.parent, directory_flags)
+        try:
+            os.fsync(directory_descriptor)
+        finally:
+            os.close(directory_descriptor)
     finally:
         if temp_name is not None:
             Path(temp_name).unlink(missing_ok=True)