tensor: add scale elementwise CUDA kernel + FFI

New csrc/ops/elementwise.cu (out[i]=in[i]*alpha), compiled by
xtrain-cuda/build.rs and exposed via launch_scale_f32 FFI, gated behind
not(no_cuda) like the existing vecadd smoke test.

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

csrc/ops/elementwise.cu

@@ -0,0 +1,17 @@
+extern "C" {
+
+// out[i] = in[i] * alpha   (in-place safe: out may alias in)
+__global__ void scale_f32(const float* in, float* out, float alpha, int n) {
+    int idx = blockIdx.x * blockDim.x + threadIdx.x;
+    if (idx < n) {
+        out[idx] = in[idx] * alpha;
+    }
+}
+
+void launch_scale_f32(const float* in, float* out, float alpha, int n, void* stream) {
+    int block = 256;
+    int grid = (n + block - 1) / block;
+    scale_f32<<<grid, block, 0, (cudaStream_t)stream>>>(in, out, alpha, n);
+}
+
+}