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/lib.rs

@@ -19,4 +19,4 @@ pub mod sample;
 mod train_loop;
 
 #[cfg(not(no_cuda))]
-pub use train_loop::{TrainConfig, TrainResult, train};
+pub use train_loop::{TrainConfig, TrainResult, eval_loss, train};