Add fixed val set (seed=123, disjoint from train)

data_catalog.json

@@ -169,6 +169,16 @@
     "status": "ACTIVE",
     "notes": "Split counts, op counts for auditing"
   },
+  {
+    "path": "fixed_train/val_5K_seed123.pt",
+    "description": "Fixed validation set: 5K examples, seed=123, disjoint from all train sets",
+    "version": "v1",
+    "n_examples": 5000,
+    "n_splits": null,
+    "seed": 123,
+    "status": "ACTIVE",
+    "notes": "Different seed than train sets (42). Used for mid-training loss monitoring."
+  },
   {
     "path": "sub_handcrafted/test.parquet",
     "description": "Handcrafted subtraction test cases",