phase 2: tensor abstraction layer

- DType enum (F32, F16, BF16) with TensorDType trait
- Shape utilities: contiguous_strides, broadcast_shape, broadcast_strides
- Storage with Arc reference counting (CPU Vec<u8> or GPU GpuBuffer)
- Device enum (Cpu, Cuda(id)) with to_device transfer
- Tensor type with strided layout: reshape, transpose, squeeze, unsqueeze
- contiguous() copies non-contiguous views to contiguous layout
- from_slice, zeros, ones constructors
- as_slice<T> for typed CPU read access, data_ptr for GPU kernel launch
- CPU↔GPU roundtrip verified
- All 27 tests pass (12 cuda + 4 shape + 11 tensor)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-tensor/Cargo.toml

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