fix: handle transition edge cases

[ShiroKSH]

Summary

• return boolean results for unknown events instead of raising a nil-index error
• stop leave callbacks after an onbefore veto and clear the transition marker on cancellation
• preserve nil callback arguments across async transitions
• correctly report an invalid event while it cancels a pending async transition
• correct initialization and async-cancellation examples

Tests

• Busted spec on LuaJIT: 23 successes, 0 failures, 0 errors, 0 pending