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>

crates/xtrain-cuda/src/ffi.rs

@@ -24,9 +24,19 @@ unsafe extern "C" {
     pub fn cudaGetErrorString(error: i32) -> *const c_char;
 }
 
-// The vector-add smoke-test kernel, compiled from csrc/test/vecadd.cu by build.rs.
-// Only linked when CUDA is actually compiled (i.e. nvcc was present).
+// GPU kernels compiled from csrc/ by build.rs. Only linked when CUDA is
+// actually compiled (i.e. nvcc was present).
 #[cfg(not(no_cuda))]
 unsafe extern "C" {
+    // Vector-add smoke test (csrc/test/vecadd.cu).
     pub fn launch_vecadd_f32(a: *const f32, b: *const f32, c: *mut f32, n: i32, stream: CudaStream);
+
+    // Elementwise scale: out[i] = in[i] * alpha (csrc/ops/elementwise.cu).
+    pub fn launch_scale_f32(
+        input: *const f32,
+        out: *mut f32,
+        alpha: f32,
+        n: i32,
+        stream: CudaStream,
+    );
 }