dropout: wire into model (residual sites) + train/eval switch + flag (T18)

Config.dropout (default 0). TinyTransformer gets a Cell<bool> training switch
(train()/eval()/with_training, default eval = safe) + a Cell<u64> step_seed bumped
once per training forward. forward_batched derives a per-layer block_seed (pure fn
of step_seed×layer) and block_forward derives two per-site seeds, inserting
ops::dropout at the attn and ffn sub-block outputs (before each residual). The
seed is a pure function of (step_seed, layer, site) so the checkpoint (T13)
recompute re-derives the same masks → grads stay exact. p=0 or eval → no dropout
node → graph bit-identical to pre-T18.

train_loop: model.train() per step (restored after eval flips to eval); eval_loss
runs model.eval(). bin/train: --dropout flag → cfg.dropout. Export/sampling run in
eval (default), so exported weights are dropout-free (xserv closed loop unaffected).

Model-level tests (dropout.rs): p=0 bit-identical to no-dropout (logits/loss/grads);
eval(p>0) == p=0 identity; train differs from eval + finite; recompute-with-dropout
grads match non-recompute (fp32 + bf16).

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

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

@@ -109,6 +109,10 @@ fn main() {
     let val_tokens: usize = flag(&args, "--val-tokens", 0);
     let eval_every: usize = flag(&args, "--eval-every", 0);
     let eval_batches: usize = flag(&args, "--eval-batches", 64);
+    // Dropout (Phase T18): residual-path dropout prob, active at training time
+    // only (inverted scaling), identity at eval/sampling/export. Default 0 = off
+    // (forward graph bit-identical to the no-dropout path).
+    let dropout: f32 = flag(&args, "--dropout", 0.0f32);
     // bf16 mixed precision (Phase T12): fp32 master weights, bf16 linears +
     // activations. Opt-in; default fp32 reproduces v0–v4 numerics.
     let bf16 = args.iter().any(|a| a == "--bf16");
@@ -149,7 +153,8 @@ fn main() {
         (corpus, None)
     };
 
-    let cfg = Config::from_arch(vocab, n_heads, head_dim, n_layers, ffn);
+    let mut cfg = Config::from_arch(vocab, n_heads, head_dim, n_layers, ffn);
+    cfg.dropout = dropout;
     println!(
         "model: dim {} layers {} heads {} head_dim {} ffn {} → core {:.3}M params \
          (+ embed/lm {:.2}M = {:.2}M total)",
@@ -183,6 +188,9 @@ fn main() {
         model = model.with_recompute(true);
         println!("activation recompute: ON (per-block gradient checkpointing)");
     }
+    if dropout > 0.0 {
+        println!("dropout: ON (p={dropout}, residual-path, train-only inverted scaling)");
+    }
 
     // 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