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/src/device.rs

@@ -0,0 +1,16 @@
+use crate::error::{self, Result};
+use crate::ffi;
+
+pub fn device_count() -> Result<i32> {
+    let mut count = 0;
+    error::check(unsafe { ffi::cudaGetDeviceCount(&mut count) })?;
+    Ok(count)
+}
+
+pub fn set_device(device: u32) -> Result<()> {
+    error::check(unsafe { ffi::cudaSetDevice(device as i32) })
+}
+
+pub fn synchronize() -> Result<()> {
+    error::check(unsafe { ffi::cudaDeviceSynchronize() })
+}