filelock: add replace_content for in-place locked rewrite

Rotation rewrites the database file while holding the exclusive lock;
flock follows the open file description across ftruncate, and on Windows
in-place rewrite avoids share-mode rename restrictions. Also update
rotation doc timestamp format to ISO basic with microseconds.

tests/test_filelock.py

@@ -0,0 +1,55 @@
+"""Tests for LockedFile low-level behaviors."""
+
+from kanta.exceptions import FileLockError
+from kanta.filelock import LockedFile
+
+
+def test_replace_content_rewrites_in_place(tmp_path):
+    path = tmp_path / "data.kantadb"
+    path.write_bytes(b"original content here")
+
+    f = LockedFile()
+    f.open(path)
+    try:
+        f.replace_content(b"new")
+        assert f.size() == 3
+        f.write(b"!")
+    finally:
+        f.close()
+
+    assert path.read_bytes() == b"new!"
+
+
+def test_replace_content_keeps_lock(tmp_path):
+    path = tmp_path / "data.kantadb"
+    path.write_bytes(b"abc")
+
+    f = LockedFile()
+    f.open(path)
+    try:
+        f.replace_content(b"xyz")
+        other = LockedFile()
+        try:
+            other.open(path)
+            raise AssertionError("second open should fail while lock is held")
+        except FileLockError:
+            pass
+    finally:
+        f.close()
+
+
+def test_replace_content_grow_and_shrink(tmp_path):
+    path = tmp_path / "data.kantadb"
+    path.write_bytes(b"x" * 100)
+
+    f = LockedFile()
+    f.open(path)
+    try:
+        f.replace_content(b"")
+        assert f.size() == 0
+        f.replace_content(b"y" * 200)
+        assert f.size() == 200
+    finally:
+        f.close()
+
+    assert path.read_bytes() == b"y" * 200