test: normalize NumPy scalars in space-time receipts

tests/validation/multigrid/spacetime_reference.py

@@ -229,6 +229,15 @@ def advance(spec, plan, state, mode="spacetime", mutation=None):
                        negative_boundary_count=negative_boundary, nonfinite_boundary_count=nonfinite_boundary)
 
 
+def write_receipt(path, result):
+    """Normalize NumPy scalars at the JSON boundary; reject other unknown types."""
+    def scalar(value):
+        if isinstance(value, np.generic):
+            return value.item()
+        raise TypeError(f"Object of type {type(value).__name__} is not JSON serializable")
+    path.write_text(json.dumps(result, indent=2, allow_nan=False, default=scalar)+"\n")
+
+
 def main():
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument("--n64", action="store_true", help="explicitly run the frozen 1024-interval experiment")
@@ -267,7 +276,7 @@ def main():
                   limits=dict(seconds=args.max_seconds,rss_mib=args.max_rss_mib),runs=[],completed=False)
     def save():
         result["resources"] = dict(seconds=time.monotonic()-started,peak_rss_mib=resource.getrusage(resource.RUSAGE_SELF).ru_maxrss/1024)
-        args.output.write_text(json.dumps(result,indent=2,allow_nan=False)+"\n")
+        write_receipt(args.output, result)
     save()
     try:
         for mode in ("constant_ivp", "spacetime"):