refactor: use Map<wrapperId, EventListener> instead of document expando properties

[adnaan]

Context

Discovered during PR #58 review.

Problem

EventDelegator and LinkInterceptor store listener references as expando properties on document:

(document as any)[`__lvt_delegated_${eventType}_${wrapperId}`] = listener;
(document as any)[`__lvt_link_intercept_${wrapperId}`] = listener;

Issues:

• Non-standard — expando properties on DOM nodes
• Invisible to TypeScript (requires as any casts)
• Risks collisions with external scripts that iterate document properties
• Harder to introspect during debugging

Proposed Fix

Use a private Map<string, EventListener> on each class.

Scope

Deferred from PR #58 to keep that PR focused. All locations using expando properties should migrate:

• dom/event-delegation.ts — __lvt_delegated_${eventType}_${wrapperId}
• dom/event-delegation.ts — __lvt_window_${eventType}_${wrapperId}
• dom/event-delegation.ts — __lvt_focus_trap_${wrapperId}
• dom/event-delegation.ts — __lvt_autofocus_observer_${wrapperId}
• dom/link-interceptor.ts — __lvt_link_intercept_${wrapperId}