Add Event Emitters for Section Transitions

[bae080311]

Summary

Introduce a robust event system allowing developers to react to section entry, exit, and active state changes, enhancing custom behavior, analytics, or UI updates.

Proposed Solution

Implement event emitters (e.g., onSectionEnter, onSectionExit, onSectionActiveChange) that dispatch events with relevant section data. This complements existing features like custom easing (#20) and debugging (#17, #24).

Acceptance Criteria

• jump-section emits events when a section becomes active/inactive.
• Events provide data like sectionId, index, direction.
• Clear documentation and examples for using the event system.

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