Revised migration context: don't store changerecord unless something was changed, and always snapshot if and after changes done.

tests/support.py

@@ -7,7 +7,7 @@ from uuid import UUID
 import msgspec
 
 from kanta.kanta import Kanta
-from kanta.structs import ChangeRecord
+from kanta.structs import ChangeRecord, Snapshot
 
 
 class User(msgspec.Struct):
@@ -89,6 +89,17 @@ def make_migrations_module(name: str, fn_name: str, fn):
     return mod
 
 
+def read_last_snapshot(path: Path, format_config) -> Snapshot | None:
+    _, serializer_cls = format_config
+    serializer = serializer_cls()
+    framer = serializer.framer_cls()
+    data = path.read_bytes()
+    payload, _, _ = framer.scan_last_snapshot(data)
+    if payload is None:
+        return None
+    return serializer.decode(payload, type=Snapshot)
+
+
 def fixed_change(action: str, diff: dict, *, version: int = 0) -> ChangeRecord:
     return ChangeRecord(
         ts=datetime(2026, 1, 1, tzinfo=UTC), a=action, v=version, diff=diff