Group table rows by model sub-family (size) instead of individual model

Changes table_group_by from "model" to "model_family" so that
general families like Llama-3.2 visually group by size (1B, 3B)
with separators and per-group best-metric highlighting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config.json

@@ -131,7 +131,7 @@
       "direction": "desc"
     }
   ],
-  "table_group_by": "model",
+  "table_group_by": "model_family",
   "model_families": {
     "Cosmos-Reason2-2B": {
       "data_file": "data/Cosmos-Reason2.csv",