Add occlusion insets for persistent overlays

[robmaceachern]

Adds Behavior.occlusionInsets so Listable can account for persistent UI that visually covers the list viewport, such as a floating bottom action bar.

Key changes:

• Adds Behavior.occlusionInsets as a semantic inset for persistent viewport occlusion.
• Applies occlusion insets to the underlying scroll view content inset, and applies the relevant axis-specific edges to scroll indicator insets.
• Combines occlusion insets with keyboard avoidance so first-responder scrolling treats the occluded area as unavailable.
• Keeps occlusion geometry separate from layout.bounds.padding, which continues to control Listable content layout.
• Relayouts synchronously when adjusted scroll insets change so visible attributes stay aligned before UIKit auto-scrolls the focused input.
• Adds unit coverage, README/changelog documentation, and a development demo for text fields behind a floating bottom overlay.

Validation

• git diff --check
• xcodebuild test -workspace Development/ListableDevelopment.xcworkspace -scheme Listable -destination 'id=31537917-EB30-47B9-BAF9-DD541083B1DA' -configuration Debug -derivedDataPath /tmp/ListablePR614DerivedData -only-testing:ListableTests/ListViewTests/test_calculateScrollViewInsets -only-testing:ListableTests/BehaviorTests/test_init
• Built, installed, and launched the demo app on Listable PR 614 iPhone 16 Pro.

Checklist

Please do the following before merging:

• Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.

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This description was drafted with AI assistance.