Add approaching-bottom pagination observer

[robmaceachern]

Describe your changes here. Please include screenshots if they're visual!

Adds a first-class onApproachingBottom observer for pagination use cases, plus a dedicated demo in the Listable development app.

What changed

• Added ListStateObserver.onApproachingBottom(within:shouldPerform:_:) to expose a built-in approaching-bottom observer for list pagination.
• Added ListScrollPositionInfo.BottomThreshold, contentSize, and isApproachingBottom(within:) to support threshold-based bottom detection.
• Added unit coverage for threshold calculations, de-duplication, re-arming after scroll/content/frame changes, and shouldPerform gating.
• Added an Approaching Bottom Pagination demo screen to the development app.
• Restored the touched files to the repo's existing formatting style so the PR diff stays focused on the new API and demo.
• Documented the new public API in CHANGELOG.md.

Testing

• mise exec -- tuist generate --no-open --path Development
• xcodebuild test -workspace /Users/robmaceachern/Development/Listable/Development/ListableDevelopment.xcworkspace -scheme Listable -destination 'platform=iOS Simulator,id=C4028877-CBF8-4816-88AD-B9C88E490FB1' -only-testing:ListableTests/UIRectEdgeTests -only-testing:ListableTests/ListStateObserverApproachingBottomTests\n\n### Checklist\n\n- [x] Ensure any public-facing changes are reflected in the changelog.

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