train: --eval-ckpt eval-only mode (v0-vs-v1 same-set val loss)

Expose eval_loss() and add a --eval-ckpt <path> branch to bin/train: load an
existing checkpoint into a model of the given arch and score it on the held-out
val split, then exit. Lets v0 and v1 be measured on the identical validation set
(the acceptance metric) without a separate eval binary.

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

crates/xtrain-train/src/bin/train.rs

@@ -167,6 +167,20 @@ fn main() {
         }
     });
 
+    // Eval-only mode: load a checkpoint and score it on the held-out val set, then
+    // exit. Used to put an EXISTING model (e.g. v0) and a new one on the same
+    // metric — the v0-vs-v1 val-loss comparison. The arch flags must match the ckpt.
+    if let Some(p) = args.iter().position(|a| a == "--eval-ckpt") {
+        let ckpt_path = PathBuf::from(args.get(p + 1).expect("--eval-ckpt <path>"));
+        xtrain_train::checkpoint::load_into(&ckpt_path, &model.params())
+            .expect("load eval checkpoint");
+        let v = valid.expect("--eval-ckpt needs --val-tokens > 0");
+        let vl = xtrain_train::eval_loss(&model, device, &v, seq_len, eval_batches);
+        println!("eval-only: {} → val loss {vl:.4}", ckpt_path.display());
+        sample_some(&model, device, &tok_path);
+        return;
+    }
+
     let tcfg = TrainConfig {
         seq_len,
         batch_size,