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>

Cargo.toml

@@ -2,9 +2,14 @@
 resolver = "2"
 members = [
     "crates/xtrain-cuda",
+    "crates/xtrain-tensor",
 ]
 
 [workspace.package]
 version = "0.1.0"
 edition = "2024"
 license = "MIT"
+
+[workspace.dependencies]
+half = "2"
+smallvec = "1"