rotation: tests, replay-base cutoff state fix, formatting

- tests/test_rotation.py covers both framers: history split, idempotent
  reopen, no-op cases, aged-out reduction to a single snapshot, internal
  snapshot validation, extension-agnostic rotated naming, and the
  disabled default.
- Fix plan_rotation to seed the cutoff state from the replay base
  snapshot (previously asserted when the base already covered the cutoff).
- docs: snapshots carry the schema version in effect at their position,
  keeping replay/migration behavior identical to the unrotated file.

docs/rotation.md

@@ -155,16 +155,18 @@ into memory by `open_and_read`; no second disk read is needed.
 5. **Rewrite the main file in place.** On the locked fd: seek to 0, truncate
    to 0, write the new content, `fdatasync`. The new content is, in order:
    1. A **snapshot of the state at the cutoff** — the replayed state after
-      applying all records with `ts < cutoff`, stamped with the **current
-      schema version**. Its `ts` is the **ts of the last pre-cutoff record**
-      (not the rotation time), and this is exactly the timestamp used in the
-      rotated filename. This snapshot is the new replay base and carries the
-      version forward so migrations are not re-run; it is always written.
+      applying all records with `ts < cutoff`, stamped with the **schema
+      version in effect at the cutoff**. Its `ts` is the **ts of the last
+      pre-cutoff record** (not the rotation time), and this is exactly the
+      timestamp used in the rotated filename. This snapshot is the new replay
+      base and carries the version forward so migrations are not re-run; it is
+      always written.
    2. The retained change records (`ts >= cutoff`), **recreated record by
       record** — no internal snapshots are carried over, even if the original
       file had many in the retained range.
    3. A **final snapshot** of the state after the last retained record,
-      stamped with the current schema version — written **only if** there were
+      stamped with the version of the last retained record — written **only
+      if** there were
       retained change records (and, in line with the existing snapshot policy
       in `kanta/snapshot.py`, only when a meaningful number of changes
       accumulated; a handful of trailing changes need not force one). If no