tensor: minimal Tensor crate over xtrain-cuda

New xtrain-tensor crate: DType (F32), shape/stride helpers, Arc-counted
host/device Storage with CPU↔CUDA copy, and a contiguous Tensor with
creation, host↔device transfer, and a scale() op driving the elementwise
kernel. GPU integration tests (host↔device roundtrip + scale correctness)
gated behind not(no_cuda); a thin build.rs emits the no_cuda cfg so the
kernel call sites compile out locally.

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

crates/xtrain-tensor/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "xtrain-tensor"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+xtrain-cuda = { path = "../xtrain-cuda" }
+half.workspace = true
+smallvec.workspace = true