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/validation/run_dfg_re20.py

@@ -108,6 +108,21 @@ def summarize(rows):
                 verdict=inside)
 
 
+def read_result(path):
+    """Cases and provenance from a result.json of either schema.
+
+    Schema 1 (receipts before RESULT_SCHEMA=2, e.g. evidence 70f6617) is a bare list of
+    cases without provenance; schema 2 is {schema, provenance, cases}. Readers that
+    indexed [0] on schema 1 must use read_result(path)["cases"][0].
+    """
+    data = json.loads(Path(path).read_text())
+    if isinstance(data, list):
+        return dict(schema=1, provenance=None, cases=data)
+    if not isinstance(data, dict) or data.get("schema") != RESULT_SCHEMA:
+        raise ValueError(f"unknown DFG Re20 result schema: {data.get('schema') if isinstance(data, dict) else type(data).__name__}")
+    return dict(schema=RESULT_SCHEMA, provenance=data["provenance"], cases=data["cases"])
+
+
 def healthy(row):
     return row["max_mach"] <= 0.2 and row["rho_min"] >= 0.5 and row["rho_max"] <= 1.5