Stamp changelog for 0.6.6 release

[Iron-Ham]

Summary

• Stamps the [Unreleased] changelog section as [0.6.6] - 2026-02-24
• Adds a fresh empty [Unreleased] section for future changes

Release contents

Added

• ListConfigurable protocol providing shared convenience APIs via protocol extension

Changed

• Deduplicated computeItemsToReconfigure and flattenSectionSnapshot across data sources
• Removed dead convenience methods from ListConfigurationBridge (now on ListConfigurable)

Fixed

• MixedListDataSource.apply(_:to:animatingDifferences:) bypassing autoReconfigure
• OutlineList.setItems not reconfiguring ContentEquatable cells (OutlineList.setItems does not reconfigure cells when content changes (ContentEquatable) #65)
• Context menus not appearing on Mac Catalyst (Context menus not appearing on Mac Catalyst — singular delegate deprecated since iOS 16 #67)

Test plan

• CI passes
• After merge, tag 0.6.6 on the merge commit