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/tests/integration.rs

@@ -7,7 +7,8 @@ fn test_device_info() {
 
     let info = device::device_info(0).expect("failed to get device info");
     println!("GPU 0: {}", info.name);
-    println!("  Memory: {} MB", info.total_memory / (1024 * 1024));
+    println!("  Total Memory: {} MB", info.total_memory / (1024 * 1024));
+    println!("  Free Memory:  {} MB", info.free_memory / (1024 * 1024));
     println!(
         "  Compute Capability: {}.{}",
         info.compute_major, info.compute_minor
@@ -17,7 +18,7 @@ fn test_device_info() {
     println!("  Warp Size: {}", info.warp_size);
     println!("  Max Threads/Block: {}", info.max_threads_per_block);
 
-    assert!(info.total_memory > 0);
+    assert!(info.total_memory > 30 * 1024 * 1024 * 1024); // 5090 has 32GB
     assert!(info.sm_count > 0);
 }