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.

docs/rotation.md

@@ -44,15 +44,16 @@ The history that aged out is preserved at:
 - The timestamp is the **ts of the last record dropped by the rotation** (see
   step 4 — the leading snapshot of the rewritten main file carries the same
   ts), not the current time. The name tells you exactly which point in history
-  the rotated file ends at. Rendered in ISO 8601 basic format with the same
-  microsecond precision as the record's ``ts`` in the database (e.g.
-  `20260902T143000.123456Z`), so the filename matches precisely the ``ts`` of
-  the final line of the rotated file and of the snapshot at the start of the
-  new file.
+  the rotated file ends at. Rendered in ISO 8601 basic format at second
+  precision (e.g. `20260902T143000Z`). The exact microsecond timestamp of the
+  cutoff remains available inside the file (it is the ``ts`` of the final
+  line of the rotated file and of the snapshot at the start of the new file);
+  a second rotation within the same second cannot occur because rotation
+  requires history to have aged past the cutoff.
 - The rotated name always ends in `.kantadb`, regardless of the original
   extension. Users may name their databases with no extension, `.kantadb`, or
   anything else (`.db`, …). Since the rotated name is derived from the *stem*,
-  all of these work uniformly: `data` → `data@20260902T143000.123456Z.kantadb`,
+  all of these work uniformly: `data` → `data@20260902T143000Z.kantadb`,
   `data.kantadb` → `data@….kantadb`, `data.db` → `data@….kantadb`.
 - Rotated files live in the same directory.
 - Collision: if a rotated file with the same name already exists (rotation
@@ -216,8 +217,9 @@ original `record_offset`, so binary checksums stay valid).
 Add keyword options to `Kanta(...)` (`kanta/kanta.py`), surfaced through
 `open()`:
 
-- `retention: timedelta | None = None` — history window to keep. `None`
-  (default) disables rotation entirely; current behavior is unchanged.
+- `retention: timedelta | int | None = None` — history window to keep; a plain
+  `int` is interpreted as a number of days. `None` (default) disables rotation
+  entirely; current behavior is unchanged.
 - `rotate_keep: int = 3` (optional, later) — how many rotated backups to
   retain; older ones are pruned at rotation time.