[lukewilliamboswell]

Summary

Closes #538.

• Add User1(unix), User2(unix), and WindowResize(unix) requests for SIGUSR1, SIGUSR2, and SIGWINCH, with the existing System.Unix witness requirement.
• Extend cooperative interruption, event coalescing, and polling through the native host and regenerated ABI.
• Move the private native-I/O wake signal to SIGURG so SIGUSR2 remains available to applications. Check cancellation and worker reuse while SIGUSR2 is caught.
• Use each Unix signal's native number for unhandled Interrupted exit statuses, preserving Ctrl-C and SIGTERM behavior.
• Cover sleep and stdin interruption, cleanup, terminal restoration, and witness rejection for the new requests.

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked --all-targets passes all 199 tests.
• ci/regenerate_glue.sh --check passes with the CI-pinned Roc nightly and matching glue source.
• python3 scripts/test_system_api.py passes 1 positive and 22 negative compiler checks.
• python3 scripts/build.py --target x64musl, then ./scripts/test.py --no-build, pass the full native suite with the CI-pinned Roc nightly, including all six new signal cases.
• Windows GNU cargo check --locked --all-targets passes when cross-compiled with Zig from Linux.
• A separate application propagating Interrupted from main! exits 130, 143, 138, 140, and 156 for SIGINT, SIGTERM, SIGUSR1, SIGUSR2, and SIGWINCH respectively on Linux.