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/ffi.rs

@@ -30,7 +30,7 @@ pub struct CudaDeviceProp {
     _pad: [u8; 4096],
 }
 
-extern "C" {
+unsafe extern "C" {
     // --- Device ---
     pub fn cudaGetDeviceCount(count: *mut i32) -> i32;
     pub fn cudaSetDevice(device: i32) -> i32;