T1: scaffold repo + Rust/CUDA build chain (vecadd smoke test)

Stand up the xtrain project skeleton: a Cargo workspace mirroring xserv's
csrc/ + crates/ layout, with a single xtrain-cuda crate that wraps the CUDA
Runtime over hand-written extern "C" FFI. build.rs compiles csrc/test/vecadd.cu
via the cc crate targeting sm_120 (RTX 5090) and links cudart.

A gated integration test runs the vector-add kernel on the GPU and asserts the
result. When nvcc is absent (local GPU-less machine), build.rs skips CUDA
compilation and sets a `no_cuda` cfg so host-side cargo check still works.

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

crates/xtrain-cuda/build.rs

@@ -0,0 +1,39 @@
+use std::env;
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(no_cuda)");
+    println!("cargo:rerun-if-changed=../../csrc/");
+
+    let cuda_path = env::var("CUDA_HOME")
+        .or_else(|_| env::var("CUDA_PATH"))
+        .unwrap_or_else(|_| "/usr/local/cuda".to_string());
+
+    // Locally there is no nvcc / GPU. Detect that and skip the CUDA build so
+    // `cargo check`/`cargo build` of host-side Rust still works. The `no_cuda`
+    // cfg makes the FFI bindings + smoke test compile (but not run) without nvcc.
+    if !nvcc_available(&cuda_path) {
+        println!("cargo:warning=nvcc not found — skipping CUDA compilation (host-only build).");
+        println!("cargo:rustc-cfg=no_cuda");
+        return;
+    }
+
+    println!("cargo:rustc-link-search=native={cuda_path}/lib64");
+    println!("cargo:rustc-link-lib=dylib=cudart");
+    println!("cargo:rustc-link-lib=dylib=cuda");
+
+    cc::Build::new()
+        .cuda(true)
+        .cudart("shared")
+        .flag("-gencode=arch=compute_120,code=sm_120")
+        .file("../../csrc/test/vecadd.cu")
+        .compile("xtrain_cuda_kernels");
+}
+
+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()
+}