test: DFG Re20 provenance refuses dirty trees; versioned receipts

Per review of aceea70e: a dirty checkout is refused (HEAD would not
describe the code that runs, and a dirty-to-dirty edit would pass the
end-of-run check). On a clean tree HEAD pins the imported helpers too.
plan.json and result.json carry schema=2; schema-1 receipts (bare list,
e.g. evidence 70f6617) are documented. Adds a dirty-refusal test.

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

tests/validation/run_dfg_re20.py

@@ -57,6 +57,8 @@ WINDOW_TSTAR = 10.0            # trailing 5 s, 21 samples
 BANDS = dict(cd=(5.57, 5.59), cl=(0.0104, 0.0110), p_diff=(0.1172, 0.1176))
 STEADY_RANGE = {k: (hi - lo) / 10 for k, (lo, hi) in BANDS.items()}
 STEADY_SLOPE = {k: (hi - lo) / 50 for k, (lo, hi) in BANDS.items()}
+# 2: {schema, provenance, cases}; the earlier receipts (schema 1) are a bare list of cases.
+RESULT_SCHEMA = 2
 DEVIATIONS = ("pressure-NEQ outlet (neq_extrap), not do-nothing", "local Zou-He inlet")
 
 _re100_config = rr.config
@@ -201,8 +203,11 @@ def provenance(source):
     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())
+    record = dict(source_mode="current_checkout", **checkout_provenance(source), src_tree=tree,
+                  runner_sha256=hashlib.sha256(Path(__file__).read_bytes()).hexdigest())
+    if record["checkout_dirty"]:  # HEAD would not describe the code that runs
+        raise RuntimeError("DFG Re20 runs require a clean checkout")
+    return record
 
 
 def main():
@@ -215,7 +220,7 @@ 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))
+    record = dict(plan(a.collision), schema=RESULT_SCHEMA, provenance=provenance(a.source))
     a.output.mkdir(parents=True, exist_ok=False)
     (a.output / "plan.json").write_text(json.dumps(record, indent=2, allow_nan=False))
     if a.plan:
@@ -230,7 +235,8 @@ def main():
     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),
+    (a.output / "result.json").write_text(json.dumps(dict(schema=RESULT_SCHEMA, provenance=after,
+                                                          cases=results),
                                                      indent=2, allow_nan=False))