Add keyboard adjustment additional insets

[robmaceachern]

Adds Listable support for keyboard avoidance when persistent overlay UI, such as a floating bottom action bar, occupies part of the keyboard-adjusted viewport.

Key changes:

• Adds Behavior.keyboardAdjustmentAdditionalInsets, applied only while the keyboard overlaps the list, so clients can reserve overlay space without replacing Listable's keyboard inset calculation.
• Folds those additional insets into the content and scroll indicator inset calculation used for first-responder visibility.
• Relayouts synchronously when keyboard-driven adjusted insets change, keeping visible layout attributes aligned before UIKit auto-scrolls the focused input.
• Promotes UIKit's non-animated first-responder scrollRectToVisible call to an animated scroll only during keyboard frame transitions.
• Tracks UITextView editing notifications for supplementary first-responder retention, matching existing UITextField behavior.
• Adds unit coverage, README/changelog documentation, and a development demo for a text area behind a floating bottom overlay.

Validation

• xcodebuild test -workspace Development/ListableDevelopment.xcworkspace -scheme ListableDevelopment-Workspace -destination 'platform=iOS Simulator,name=iPhone 16 Pro,OS=18.1'
• git diff --check

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.