kernel-gpu: two wavefront tests fail on Apple silicon (macOS 27)

[ecto]

cargo test -p vcad-kernel-gpu wavefront::tests::gpu_compacted fails deterministically on an M-series Mac running macOS 27:

• wavefront::tests::gpu_compacted_matches_cpu_on_walled_grid (wavefront.rs:916)
• wavefront::tests::gpu_compacted_matches_cpu_multi_source_random_obstacles (wavefront.rs:946)

The other 19 tests in the crate pass, and CI (Linux, no GPU) is green. The crate has no dependency on the boolean kernel; noticed while running the full workspace for #890.

🤖 Filed by Claude Code