T1: scaffold repo + Rust/CUDA build chain (vecadd smoke test)

Stand up the xtrain project skeleton: a Cargo workspace mirroring xserv's
csrc/ + crates/ layout, with a single xtrain-cuda crate that wraps the CUDA
Runtime over hand-written extern "C" FFI. build.rs compiles csrc/test/vecadd.cu
via the cc crate targeting sm_120 (RTX 5090) and links cudart.

A gated integration test runs the vector-add kernel on the GPU and asserts the
result. When nvcc is absent (local GPU-less machine), build.rs skips CUDA
compilation and sets a `no_cuda` cfg so host-side cargo check still works.

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

README.md

@@ -0,0 +1,50 @@
+# xtrain
+
+A from-scratch **Rust + CUDA** LLM **training** engine — the sibling of
+[xserv](https://github.com/) (the inference side). GPU-first.
+
+The goal is to learn the full training-systems stack by hand: autograd / backward
+passes / optimizers (AdamW) / the training loop / distributed logic. Heavy lifting
+is borrowed where it makes sense (GEMM → cuBLAS after a hand-written version,
+multi-GPU comms → NCCL, tokenizer → reused from xserv), but the core is written
+from scratch. The target architecture is a tiny modern transformer
+(RoPE + RMSNorm + SwiGLU, ~1–30M params) whose forward aligns with xserv's Qwen3,
+so the backward passes map one-to-one onto xserv's existing forward kernels and
+trained weights can flow back into xserv.
+
+## Status
+
+Bootstrapping (P0). This repo currently contains only the project skeleton and a
+working Rust↔CUDA build chain, verified by a trivial vector-add CUDA kernel.
+
+## Layout
+
+```
+xtrain/
+├── Cargo.toml              # workspace
+├── csrc/                   # CUDA sources (.cu)
+│   └── test/vecadd.cu      # trivial element-wise vector-add (smoke test)
+└── crates/
+    └── xtrain-cuda/        # CUDA Runtime FFI + build.rs (nvcc → sm_120)
+        ├── build.rs        # compiles csrc/*.cu via the `cc` crate, links cudart
+        ├── src/            # ffi / error / device / memory
+        └── tests/          # vecadd smoke test
+```
+
+The build mirrors xserv's approach: `build.rs` invokes `nvcc` (via the `cc` crate)
+to compile `csrc/*.cu` targeting `sm_120` (RTX 5090) and links them into the Rust
+crate over hand-written `extern "C"` FFI.
+
+## Building & testing
+
+CUDA compilation and execution happen on a GPU box (dash5, 8× RTX 5090, sm_120):
+
+```sh
+export PATH=/usr/local/cuda/bin:$HOME/.cargo/bin:$PATH
+cargo build
+cargo test -p xtrain-cuda -- --nocapture   # runs the vecadd smoke test
+```
+
+On a machine without `nvcc`/GPU, `build.rs` detects the missing toolchain, skips
+CUDA compilation, and sets a `no_cuda` cfg — so host-side `cargo check` still
+works (the GPU smoke test is compiled out).