Issues #632 and #636 (PRs #651 and #653) both stem from the same root cause: async
threads spawned via async() can outlive the Dispatch_Engine, accessing shared state
(global objects map, type maps) after it has been destroyed. The fix moves async()
registration from the stdlib module into ChaiScript_Basic, where spawned threads are
tracked via Dispatch_Engine. The engine's destructor now joins all outstanding async
threads before destroying shared data structures.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
