#292 Merge classic fixes with split-tree observation fix

This integration branch preserves the PR #291 SwiftUI observation fix while merging the current Bonsplit main history, including the divider double-click delegation/release corrections from classic cmux main.

It is required by cmux PR https://github.com/manaflow-ai/cmux/pull/22901 before the parent gitlink can advance safely.

Changelog

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Summary by CodeRabbit

• New Features
  • Delegate handlers can respond to divider double-clicks; unhandled clicks retain native behavior.
  • An optional diagnostic callback reports sustained tab-bar rendering activity, including elapsed time, render count, tab count, and pane count.
• Improvements
  • Tab context menus evaluate their state when opened, keeping displayed information current.
  • Split-view updates and divider interactions behave more reliably during repeated updates and drag sessions.
  • Shortcut hints reflect the configured modifier when appropriate, while avoiding unnecessary changes to visible hints.