DOM event system: addEventListener, dispatch, bubbling #281

Implements issue 3mhyfnijhmc2g

Summary #

• Event constructor: new Event(type, options) with bubbles/cancelable support
• Event methods: preventDefault(), stopPropagation(), stopImmediatePropagation()
• EventTarget methods on all node wrappers and document: addEventListener, removeEventListener, dispatchEvent
• Full event propagation: capture phase → at-target → bubble phase
• Support for capture option (boolean or options object with capture field)
• Support for once option (auto-remove after first invocation)
• Event listener callbacks registered as GC roots
• 20 new tests covering all acceptance criteria

Phase 11 — DOM-JS Bindings (issue 4 of 8)