Stop propagation makes events non-reusable #18

Why: once a listener calls stop_propagation(), the event instance stays stopped. Dispatching it again skips every listener after the first one, so an event instance can't be reused across dispatches.

What: an opt-in Event.reset() that clears propagation_stopped. Default behaviour unchanged. Implementation: #23.

Acceptance criteria

• Event.reset() clears propagation_stopped; a reset event dispatched again reaches every listener.
• Without reset(), re-dispatching a stopped event behaves as today.
• docs/propagation.rst documents reusing an event with reset().
• CHANGELOG.rst lists Event.reset() under Unreleased / Added.

Out of scope

• Resetting automatically at every dispatch: breaking, tracked for 3.0 in 3.0: reset propagation_stopped at the start of every dispatch #24.
• The IEvent protocol: dispatchers never call reset().

– 🤖 po