fix(process-pool): bound shutdown latency

[dexhunter]

Fixes #297.

Summary

• prevent ProcessParallelController.stop() from waiting indefinitely for running process-pool work
• use the public worker-termination API on Python 3.14 and a bounded compatibility path on Python 3.10-3.13
• cancel queued work, terminate running workers, and escalate survivors to kill() after a bounded wait
• add regression coverage for stuck workers, cleanup idempotence, and kill escalation

Why

ProcessPoolExecutor future.cancel() cannot cancel a task that is already running. The existing shutdown(wait=True) therefore waits for that task to finish, which matches the 27+ second and multi-hour shutdown hangs reported in #297.

The new path captures only this executor's workers, requests immediate shutdown, waits for those processes to exit, and escalates only workers that survive termination. Unrelated processes are not touched.

Performance

A strict scaled reproduction measures process_pool_shutdown_ms across multiple task durations with correctness gates for completed results, queued/running futures, worker cleanup, repeated stop(), post-return side effects, and unrelated processes.

Variant	Metric
Current main, Python 3.10 median	1496.85 ms
Cleaned patch, Python 3.10 median	3.62 ms
Cleaned patch, Python 3.14 median	2.31 ms

That is about a 99.76% reduction (about 414x) on the minimum supported Python version. A separate 30-second worker stopped in 3.48 ms with no live worker left behind. Idle-pool shutdown remained effectively unchanged (3.40 ms before, 3.58 ms after).

Supplementary autoresearch: 20-step public dashboard.

Validation

• Python 3.10: python -m unittest discover tests (392 passed)
• Python 3.14: python -m unittest discover tests (392 passed)
• Python 3.10: focused process-parallel tests (8 passed)
• Python 3.14: focused process-parallel tests (8 passed)
• strict shutdown evaluator on Python 3.10 and Python 3.14
• Black on both touched files
• git diff --check

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