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>

csrc/test/vecadd.cu

@@ -0,0 +1,16 @@
+extern "C" {
+
+__global__ void vecadd_f32(const float* a, const float* b, float* c, int n) {
+    int idx = blockIdx.x * blockDim.x + threadIdx.x;
+    if (idx < n) {
+        c[idx] = a[idx] + b[idx];
+    }
+}
+
+void launch_vecadd_f32(const float* a, const float* b, float* c, int n, void* stream) {
+    int block = 256;
+    int grid = (n + block - 1) / block;
+    vecadd_f32<<<grid, block, 0, (cudaStream_t)stream>>>(a, b, c, n);
+}
+
+}