test+bins: flash grad-check, flash==composed, PyTorch parity, --flash flag

autograd: flash_attention_batched_bwd (dQ/dK/dV finite-diff, seq>tile)
+ flash_matches_composed_fwd. model/tests/flash.rs: flash==composed
on-vs-off (logits/loss/every param grad), fp32 + bf16. parity_dump:
XTRAIN_PARITY_FLASH dumps the flash path for the same parity.py oracle
(PyTorch SDPA parity at B>1). train + train_ddp get the --flash flag.

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

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

@@ -116,6 +116,9 @@ fn main() {
     // exact grads, lower peak activation memory (lets dim1024 batch32 fit). Opt-in;
     // default off stores every activation (unchanged numerics).
     let recompute = args.iter().any(|a| a == "--recompute");
+    // Fused flash-attention (Phase T14): single fused SDPA kernel, online softmax,
+    // no materialized [bh,S,S] scores. Opt-in; default off keeps the composed path.
+    let flash = args.iter().any(|a| a == "--flash");
     let ckpt: PathBuf = PathBuf::from(
         args.iter()
             .position(|a| a == "--ckpt")
@@ -183,6 +186,10 @@ fn main() {
         model = model.with_recompute(true);
         println!("activation recompute: ON (per-block gradient checkpointing)");
     }
+    if flash {
+        model = model.with_flash(true);
+        println!("flash-attention: ON (fused SDPA kernel, no materialized scores)");
+    }
 
     // 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