[Qwejay]

Summary

• Serialize stop, pause, and resume state transitions under the engine thread lock.
• Ensure stop atomically signals workers, releases paused workers, and clears pending nodes.
• Add regression tests for stop queue cleanup and pause/resume state transitions.

Validation

• Latest GitHub Actions run completed source compilation and the full regression-test step successfully (19 tests total).
• Windows GUI, full engine stress, node-by-node, and exported EXE tests remain necessary.