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/Cargo.toml

@@ -4,7 +4,7 @@ version.workspace = true
 edition.workspace = true
 
 [build-dependencies]
-cc = { version = "1", features = ["cuda"] }
+cc = "1"
 
 [dev-dependencies]
 rand = "0.9"