test: record checkout provenance in DFG Re20 receipts

plan.json and result.json now carry HEAD, the dirty flag, the src tree
hash and the runner hash, labelled current_checkout; result.json is
refused if the checkout changed during the run. Numerics, sampling and
the frozen steady rule are unchanged.

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

tests/unit/test_dfg_re20.py

@@ -75,5 +75,15 @@ class Re20Contract(unittest.TestCase):
         self.assertEqual((out["status"], out["verdict"]), ("INCOMPLETE", None))
 
 
+class Re20Provenance(unittest.TestCase):
+    def test_records_current_checkout(self):
+        record = v.provenance(rr.DEFAULT_SOURCE)
+        self.assertEqual(record["source_mode"], "current_checkout")
+        self.assertEqual(len(record["checkout_revision"]), 40)
+        self.assertEqual(len(record["src_tree"]), 40)
+        self.assertEqual(len(record["runner_sha256"]), 64)
+        self.assertIn("checkout_dirty", record)
+
+
 if __name__ == "__main__":
     unittest.main()

tests/validation/run_dfg_re20.py

@@ -194,6 +194,17 @@ def plan(collision="SRT"):
                 label="compared against the official reference bands (non-canonical neq_extrap outlet, zou_he_local inlet); not a full benchmark certification")
 
 
+def provenance(source):
+    """Current checkout identity: HEAD, dirty flag, src tree and this runner's hash."""
+    import hashlib
+    import subprocess
+    from source_provenance import checkout_provenance
+    tree = subprocess.check_output(["git", "-C", str(source), "rev-parse", "HEAD:src"],
+                                   text=True).strip()
+    return dict(source_mode="current_checkout", **checkout_provenance(source), src_tree=tree,
+                runner_sha256=hashlib.sha256(Path(__file__).read_bytes()).hexdigest())
+
+
 def main():
     parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
     parser.add_argument("--case", choices=["refined", "uniform", "both"], default="refined")
@@ -204,8 +215,9 @@ def main():
     parser.add_argument("--max-wall-seconds", type=float)
     a = parser.parse_args()
     a.source = a.source.resolve()
+    record = dict(plan(a.collision), provenance=provenance(a.source))
     a.output.mkdir(parents=True, exist_ok=False)
-    (a.output / "plan.json").write_text(json.dumps(plan(a.collision), indent=2, allow_nan=False))
+    (a.output / "plan.json").write_text(json.dumps(record, indent=2, allow_nan=False))
     if a.plan:
         return
     if not a.max_wall_seconds or a.max_wall_seconds <= 0 or not os.environ.get("CUDA_VISIBLE_DEVICES"):
@@ -215,7 +227,11 @@ def main():
     a.deadline = time.monotonic() + a.max_wall_seconds
     cases = ["refined", "uniform"] if a.case == "both" else [a.case]
     results = [run_case(a, h, case) for case in cases]
-    (a.output / "result.json").write_text(json.dumps(results, indent=2, allow_nan=False))
+    after = provenance(a.source)
+    if after != record["provenance"]:
+        raise RuntimeError("Source checkout changed during the run")
+    (a.output / "result.json").write_text(json.dumps(dict(provenance=after, cases=results),
+                                                     indent=2, allow_nan=False))
 
 
 if __name__ == "__main__":