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>

crates/xserv-cuda/src/error.rs

@@ -0,0 +1,43 @@
+use crate::ffi;
+use std::ffi::CStr;
+use std::fmt;
+
+#[derive(Debug)]
+pub enum CudaError {
+    OutOfMemory,
+    InvalidDevice,
+    Raw { code: i32, message: String },
+}
+
+impl fmt::Display for CudaError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            CudaError::OutOfMemory => write!(f, "CUDA out of memory"),
+            CudaError::InvalidDevice => write!(f, "CUDA invalid device"),
+            CudaError::Raw { code, message } => write!(f, "CUDA error {code}: {message}"),
+        }
+    }
+}
+
+impl std::error::Error for CudaError {}
+
+pub type Result<T> = std::result::Result<T, CudaError>;
+
+pub(crate) fn check(code: i32) -> Result<()> {
+    if code == ffi::CUDA_SUCCESS {
+        return Ok(());
+    }
+    let message = unsafe {
+        let ptr = ffi::cudaGetErrorString(code);
+        if ptr.is_null() {
+            "unknown error".to_string()
+        } else {
+            CStr::from_ptr(ptr).to_string_lossy().into_owned()
+        }
+    };
+    Err(match code {
+        ffi::CUDA_ERROR_OUT_OF_MEMORY => CudaError::OutOfMemory,
+        101 => CudaError::InvalidDevice,
+        _ => CudaError::Raw { code, message },
+    })
+}