Fix twoslash workers hanging: force exit and add timeout/stderr logging

The worker process never exited because createTwoslasher holds a TS
language service with open handles and neither it nor shiki expose
dispose(). Added process.exit(0) after final output, a 5-minute
timeout per worker, and stderr forwarding so crashes are visible.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>