test: versioned reader for DFG Re20 result.json (schema 1 and 2)

read_result() returns cases/provenance for the old bare-list receipts and
the new schema-2 object, and rejects unknown schemas. Existing evidence
(e.g. 70f6617) and its digest scripts stay valid for schema 1; queue and
digest scripts must use read_result(...)['cases'] for new receipts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_dfg_re20.py

@@ -85,6 +85,23 @@ class Re20Provenance(unittest.TestCase):
         self.assertFalse(record["checkout_dirty"])
         self.assertEqual(v.RESULT_SCHEMA, 2)
 
+    def test_reader_accepts_both_schemas_and_rejects_unknown(self):
+        import json
+        import tempfile
+        from pathlib import Path
+        case = dict(case="refined", status="NOT_STEADY", verdict=None, rows=[])
+        with tempfile.TemporaryDirectory() as d:
+            old, new, bad = Path(d, "old.json"), Path(d, "new.json"), Path(d, "bad.json")
+            old.write_text(json.dumps([case]))
+            new.write_text(json.dumps(dict(schema=2, provenance=dict(src_tree="a" * 40),
+                                           cases=[case])))
+            bad.write_text(json.dumps(dict(schema=3, cases=[case])))
+            self.assertEqual(v.read_result(old), dict(schema=1, provenance=None, cases=[case]))
+            self.assertEqual(v.read_result(new)["cases"][0], case)
+            self.assertEqual(v.read_result(new)["provenance"]["src_tree"], "a" * 40)
+            with self.assertRaisesRegex(ValueError, "unknown DFG Re20 result schema"):
+                v.read_result(bad)
+
     def test_dirty_checkout_is_refused(self):
         from unittest.mock import patch
         with patch("source_provenance.subprocess.check_output",