Add required-features to CUDA-only wavesim binaries

Mark test_cuda, bench_packed, and verify_packed binaries as requiring
the "cuda" feature. This prevents build failures when building wavesim
without CUDA support (CPU-only builds).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mivertowski

crates/ringkernel-wavesim/Cargo.toml

@@ -73,14 +73,17 @@ path = "src/bin/full_benchmark.rs"
 [[bin]]
 name = "test_cuda"
 path = "src/bin/test_cuda.rs"
+required-features = ["cuda"]
 
 [[bin]]
 name = "bench_packed"
 path = "src/bin/bench_packed.rs"
+required-features = ["cuda"]
 
 [[bin]]
 name = "verify_packed"
 path = "src/bin/verify_packed.rs"
+required-features = ["cuda"]
 
 [[example]]
 name = "educational_modes"