phase 0+1: project scaffold + xserv-cuda crate

- Cargo workspace with xserv-cuda crate
- CUDA FFI bindings (cudart: memory, stream, device, error)
- GpuBuffer RAII wrapper with H2D/D2H/D2D copy
- CudaStream wrapper with RAII Drop
- CachingAllocator with size-bucketed free lists
- PinnedBuffer for page-locked host memory
- Device info query via cudaDeviceGetAttribute
- Vector-add CUDA kernel smoke test
- Integration test suite (11 tests)
- build.rs: cc crate compiles .cu for SM 12.0
- sync-and-build.sh for remote build on dash5
- Roadmap doc (docs/00-roadmap.md) and Phase 0+1 design doc

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

Cargo.toml

@@ -0,0 +1,14 @@
+[workspace]
+resolver = "2"
+members = [
+    "crates/xserv-cuda",
+]
+
+[workspace.package]
+version = "0.1.0"
+edition = "2024"
+license = "MIT"
+
+[workspace.dependencies]
+half = "2"
+smallvec = "1"