perf(runner): use 'wait -n' for parallel job slot on Bash 4.3+

[Chemaclass]

Summary

• Add bashunit::runner::_supports_wait_n version probe.
• When Bash supports wait -n (4.3+), wait_for_job_slot blocks until any child exits instead of polling jobs -r on a 50–200ms cadence.
• Bash 3.x keeps the existing adaptive-poll path unchanged — full back-compat preserved.

Why

• Eliminates sleep-induced wakeup latency every time a parallel test finishes.
• Cuts jobs -r | wc -l subshell invocations on the hot scheduling path.

Test plan

• tests/unit/parallel_test.sh extended with:
  • test_supports_wait_n_matches_running_bash_version
  • test_wait_for_job_slot_releases_when_background_job_finishes (exercises both paths)
• ./bashunit tests/unit/ green (816 passed)
• ./bashunit --parallel tests/ green (1051 passed, 46s — was 48s baseline)
• make sa clean
• make lint clean
• Bash 3.0+ fallback path preserved

Closes #667