phase 0+1: fix Rust 2024 edition compat + memory query

- unsafe extern "C" blocks (Rust 2024 requirement)
- unsafe blocks inside unsafe fn bodies
- Use cudaMemGetInfo for accurate GPU memory reporting
- Remove cc "cuda" feature (doesn't exist, built-in)
- All 12 tests pass on RTX 5090 (CC 12.0, 170 SMs, 32GB)

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

crates/xserv-cuda/src/memory.rs

@@ -48,26 +48,30 @@ impl GpuBuffer {
     /// Safety: `src` must remain valid until the stream operation completes.
     pub unsafe fn copy_from_host_async(&mut self, src: &[u8], stream: &CudaStream) -> Result<()> {
         assert!(src.len() <= self.len);
-        error::check(ffi::cudaMemcpyAsync(
-            self.ptr,
-            src.as_ptr(),
-            src.len(),
-            ffi::CUDA_MEMCPY_H2D,
-            stream.as_raw(),
-        ))
+        unsafe {
+            error::check(ffi::cudaMemcpyAsync(
+                self.ptr,
+                src.as_ptr(),
+                src.len(),
+                ffi::CUDA_MEMCPY_H2D,
+                stream.as_raw(),
+            ))
+        }
     }
 
     /// Async copy from device to host on the given stream.
     /// Safety: `dst` must remain valid until the stream operation completes.
     pub unsafe fn copy_to_host_async(&self, dst: &mut [u8], stream: &CudaStream) -> Result<()> {
         assert!(dst.len() <= self.len);
-        error::check(ffi::cudaMemcpyAsync(
-            dst.as_mut_ptr(),
-            self.ptr,
-            dst.len(),
-            ffi::CUDA_MEMCPY_D2H,
-            stream.as_raw(),
-        ))
+        unsafe {
+            error::check(ffi::cudaMemcpyAsync(
+                dst.as_mut_ptr(),
+                self.ptr,
+                dst.len(),
+                ffi::CUDA_MEMCPY_D2H,
+                stream.as_raw(),
+            ))
+        }
     }
 
     /// Copy from another GPU buffer (D2D).