Add configurable dropout to ResBlocks

Wire a dropout hyperparameter (default 0.1) through the config, model,
training pipeline, and CLI. Persisted in saved model_config so checkpoints
reconstruct the architecture correctly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/train.py

@@ -18,6 +18,7 @@ def main() -> None:
     parser.add_argument("--hidden-dim", type=int)
     parser.add_argument("--n-blocks", type=int)
     parser.add_argument("--emb-dim", type=int)
+    parser.add_argument("--dropout", type=float, help="Dropout probability in ResBlocks (default: 0.1)")
     parser.add_argument("--val-fraction", type=float)
     parser.add_argument("--seed", type=int, help="Random seed for reproducibility")
     parser.add_argument("--validate-every", type=int,
@@ -37,6 +38,7 @@ def main() -> None:
     }.items() if v is not None}
     cli_model = {k: v for k, v in {
         "hidden_dim": args.hidden_dim, "n_blocks": args.n_blocks, "emb_dim": args.emb_dim,
+        "dropout": args.dropout,
     }.items() if v is not None}
     config_path = Path(args.config) if args.config else None
     cfg = gconfig.merge_cli_overrides(gconfig.DEFAULT_CONFIG, config_path, cli_train, cli_model)