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>

csrc/test/vecadd.cu

@@ -0,0 +1,16 @@
+extern "C" {
+
+__global__ void vecadd_f32(const float* a, const float* b, float* c, int n) {
+    int idx = blockIdx.x * blockDim.x + threadIdx.x;
+    if (idx < n) {
+        c[idx] = a[idx] + b[idx];
+    }
+}
+
+void launch_vecadd_f32(const float* a, const float* b, float* c, int n, void* stream) {
+    int block = 256;
+    int grid = (n + block - 1) / block;
+    vecadd_f32<<<grid, block, 0, (cudaStream_t)stream>>>(a, b, c, n);
+}
+
+}