test: real conflict-refresh assertions; AST-exact probe guard

- task-instances matrix test asserts same row id + rewritten due_time
  + pre-refresh None (the vacuous refreshed_same_id walrus is gone)
- compensating guard parses the two sanctioned functions' ASTs: each
  must contain its raw INSERT (children NOT NULL, admin_users PK),
  lexically inside a pytest.raises block, and no other raw SQL may
  name these tables anywhere else in the matrix file

tests/test_dao_matrix.py

@@ -276,20 +276,22 @@ def test_matrix_task_instances_crud_and_constraints(tmp_path) -> None:
         # insert (seeded) + read
         fetched = await w.instances.get(w.definition.id, D1)
         assert fetched == w.instance
-        # update path: conflict refresh on the OPEN instance
+        # update path: conflict refresh on the OPEN instance — same row
+        # id, snapshot columns (due_time) actually rewritten.
         refreshed = await w.instances.upsert(
             w.definition.id, w.child.id, D1, _stamp(), due_time="17:00"
         )
-        assert refreshed_same_id(fetched, refreshed := refreshed_same_id)
+        assert refreshed.id == fetched.id, (
+            "the conflict path must refresh the existing row in place"
+        )
+        assert refreshed.due_time == "17:00"
+        assert fetched.due_time is None
+        assert await w.instances.get_by_id(fetched.id) is not None
         return True
 
     assert _with_world(tmp_path / "m-instances.db")(_body) is True
 
 
-def refreshed_same_id(original, replacement):
-    return replacement
-
-
 def test_matrix_task_instances_constraints(tmp_path) -> None:
     async def _body(w: World):
         # D20 has no instance yet: a raw insert succeeds, then an