docs: add TabControl migration guide

[Copilot]

Added comprehensive migration guide for the TabControl refactoring that changed event naming and close button visibility control.

Changes

• Migration guide: Created docs/TabControl-Migration-Guide.md documenting breaking changes and upgrade path
  • Event system: TabItemClosing → TabCloseRequested, new TabItemHelper.CloseRequested
  • Close button visibility: IsAddTabButtonVisible → per-tab IsClosable property
  • Manual tab removal now required (no automatic removal after event)
• Code examples: Before/after patterns for XAML, code-behind, and MVVM scenarios
• Migration checklist: Quick reference for developers upgrading

Example

Before:

<TabControl ui:TabControlHelper.TabItemClosing="OnClosing" 
            ui:TabControlHelper.IsAddTabButtonVisible="False">

After:

<TabControl ui:TabControlHelper.TabCloseRequested="OnCloseRequested">
    <TabItem Header="Closable" />
    <TabItem Header="Fixed" ui:TabItemHelper.IsClosable="False" />
</TabControl>

private void OnCloseRequested(object sender, TabViewTabCloseRequestedEventArgs e)
{
    // Manual removal now required
    ((TabControl)sender).Items.Remove(e.Tab);
}

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