feat: emit swap_cancelled event on successful cancel_swap - #97

- Add SwapCancelledEvent contracttype with swap_id and canceller fields
- cancel_swap gains a canceller: Address param and calls env.events().publish
  with topic swp_cncld after the state write — event only fires on success
- Add 4 tests:
  - test_cancel_pending_swap_emits_event: asserts topic and payload fields
  - test_cancel_accepted_swap_emits_event: same for Accepted state
  - test_cancel_completed_swap_fails_no_event: panics before event fires
  - test_no_cancel_event_when_swap_completed_normally: zero events on reveal

Closes AtomicIP#19