rotation: second-precision rotated filenames, int-days retention

- Rotated filename timestamp is now ISO basic at second precision
  (20260902T143000Z); the exact cutoff ts remains inside the files.
- Kanta(retention=N) accepts a plain int as number of days.

tests/test_rotation.py

@@ -77,7 +77,7 @@ async def test_rotation_splits_history(tmp_path, format_config):
 
     # Rotated file holds exactly the dropped history, ending at the last
     # dropped record whose ts matches the filename.
-    stamp = (T0 - 40 * DAY).strftime("%Y%m%dT%H%M%S.%fZ")
+    stamp = (T0 - 40 * DAY).strftime("%Y%m%dT%H%M%SZ")
     assert rotated[0].name == f"data@{stamp}.kantadb"
     rrecords = read_all(rotated[0], format_config)
     assert [r.a for r in rrecords] == ["bootstrap", "day-40"]
@@ -181,10 +181,24 @@ async def test_rotated_naming_normalizes_extension(tmp_path, format_config, name
     async with kanta:
         pass
 
-    stamp = (T0 - 40 * DAY).strftime("%Y%m%dT%H%M%S.%fZ")
+    stamp = (T0 - 40 * DAY).strftime("%Y%m%dT%H%M%SZ")
     assert (tmp_path / f"data@{stamp}.kantadb").exists()
 
 
+async def test_retention_accepts_int_days(tmp_path, format_config):
+    path = tmp_path / "data.kantadb"
+    await write_history(path, format_config, days=[-40, -5])
+
+    cell = [T0]
+    kanta = make_kanta(path, Data, format_config, retention=30)
+    kanta.clock(make_clock(cell))
+    async with kanta:
+        assert kanta.data.counter == 2
+
+    assert len(rotated_files(path)) == 1
+    assert [c.a for c in read_changes(path, format_config)] == ["day-5"]
+
+
 async def test_rotation_disabled_by_default(tmp_path, format_config):
     path = tmp_path / "data.kantadb"
     await write_history(path, format_config, days=[-40, -5])