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/src/lib.rs

@@ -0,0 +1,15 @@
+//! Minimal tensor abstraction for xtrain (Phase T2).
+//!
+//! Provides a `DType`, shape/stride helpers, reference-counted host/device
+//! `Storage`, and a `Tensor` with creation, host↔device transfer, and one
+//! elementwise CUDA op (`scale`) wired end-to-end.
+
+pub mod dtype;
+pub mod shape;
+pub mod storage;
+pub mod tensor;
+
+pub use dtype::{DType, TensorDType};
+pub use shape::Dims;
+pub use storage::{Device, Storage};
+pub use tensor::Tensor;