model: tiny RoPE+RMSNorm+SwiGLU transformer + overfit test

New crate xtrain-model: a from-scratch decoder built entirely from the
autodiff op set.
- Config (tiny: dim=32, 2 layers, 2 heads, head_dim=16, ffn=64).
- TinyTransformer: embedding -> N x {pre-RMSNorm -> multi-head causal
  attention (RoPE, additive causal mask, per-head SDPA) -> residual;
  pre-RMSNorm -> SwiGLU MLP -> residual} -> final RMSNorm -> LM head.
  x@W weight convention (engine GEMM is plain A@B); dim=n_heads*head_dim.
- params()/zero_grad-able leaves for the optimizer; param_to_host export.
- overfit test: char-level bring-up (embedded text -> vocab -> shifted
  targets), minimal hand-written GD (p -= lr*grad) memorises one fixed
  batch -> loss ~0 + greedy argmax matches targets. End-to-end fwd+bwd
  correctness signal. Gated #![cfg(not(no_cuda))].

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

crates/xtrain-model/build.rs

@@ -0,0 +1,26 @@
+use std::env;
+use std::path::Path;
+use std::process::Command;
+
+// Same per-crate convention as the other crates: this crate's tiny-transformer
+// forward/backward calls GPU ops (via xtrain-autodiff / xtrain-tensor), so it
+// gates GPU code + tests behind `not(no_cuda)`. cfg does not propagate across
+// crates, so each crate re-detects nvcc. No CUDA is compiled here.
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(no_cuda)");
+
+    let cuda_path = env::var("CUDA_HOME")
+        .or_else(|_| env::var("CUDA_PATH"))
+        .unwrap_or_else(|_| "/usr/local/cuda".to_string());
+
+    if !nvcc_available(&cuda_path) {
+        println!("cargo:rustc-cfg=no_cuda");
+    }
+}
+
+fn nvcc_available(cuda_path: &str) -> bool {
+    if Command::new("nvcc").arg("--version").output().is_ok() {
+        return true;
+    }
+    Path::new(&format!("{cuda_path}/bin/nvcc")).exists()
+}