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.

tests/test_artifacts.py

@@ -1,4 +1,6 @@
 import json
+import os
+import stat
 from pathlib import Path
 
 from k1link.artifacts import write_json_atomic
@@ -9,3 +11,23 @@ def test_write_json_atomic(tmp_path: Path) -> None:
     write_json_atomic(output, {"value": "тест"})
     assert json.loads(output.read_text(encoding="utf-8")) == {"value": "тест"}
     assert not list(output.parent.glob("*.tmp"))
+
+
+def test_write_json_atomic_flushes_file_and_parent_directory(
+    tmp_path: Path,
+    monkeypatch,
+) -> None:
+    output = tmp_path / "nested" / "artifact.json"
+    flushed_kinds: list[str] = []
+    real_fsync = os.fsync
+
+    def observing_fsync(descriptor: int) -> None:
+        mode = os.fstat(descriptor).st_mode
+        flushed_kinds.append("directory" if stat.S_ISDIR(mode) else "file")
+        real_fsync(descriptor)
+
+    monkeypatch.setattr(os, "fsync", observing_fsync)
+
+    write_json_atomic(output, {"durable": True})
+
+    assert flushed_kinds == ["file", "directory"]