test(cli): stabilize macOS dispatch cancellation

[wgqqqqq]

Summary

• keep the dispatch cancellation safety coverage that rejects SIGKILL after the verified process-group leader exits
• make the shell leader block in its built-in wait command so TERM traps run immediately
• remove the runner-dependent timing window caused by repeatedly launching external sleep processes

Root cause

POSIX shells may defer a TERM trap while waiting for an external command. On slower macOS runners, the one-second process-group grace period could expire while the shell leader still appeared alive, so the production cancellation path correctly escalated and the test incorrectly failed. The shell built-in wait provides a deterministic handshake while the TERM-resistant child keeps the process group alive.

Verification

• focused cancellation test passed 50 consecutive runs on macOS
• cargo test --locked -p bitfun-cli -p bitfun-acp
• pnpm run check:repo-hygiene
• git diff --check

Remote scenarios

• Detached Dispatch: test-only fixture stabilization; production cancellation behavior is unchanged
• Remote workspace, Remote control, and Peer Device Mode are unchanged