Add CacheDocumentTabContent option to Browser/Fluent/Simple themes and document usage

[danwalmsley]

Summary

This PR adds an opt-in CacheDocumentTabContent theme option to Dock themes so document tab content can stay alive across tab switches (instead of being recreated each time).

Supported themes:

• BrowserTabTheme
• DockFluentTheme
• DockSimpleTheme

What changed

• Added CacheDocumentTabContent direct property to:
  • /Volumes/SSD/repos/StackWich/Dock/src/Dock.Avalonia.Themes.Browser/Styles/BrowserTabTheme.axaml.cs
  • /Volumes/SSD/repos/StackWich/Dock/src/Dock.Avalonia.Themes.Fluent/DockFluentTheme.axaml.cs
  • /Volumes/SSD/repos/StackWich/Dock/src/Dock.Avalonia.Themes.Simple/DockSimpleTheme.axaml.cs
• Split Fluent DocumentControl templates into:
  • DockDocumentControlSingleContentTemplate (default behavior, recycling template path)
  • DockDocumentControlCachedContentTemplate (cached content host for tabs, no ControlRecyclingDataTemplate)
  • File: /Volumes/SSD/repos/StackWich/Dock/src/Dock.Avalonia.Themes.Fluent/Controls/DocumentControl.axaml
• Added ReferenceEqualsMultiConverter for active-tab visibility checks in cached mode:
  • /Volumes/SSD/repos/StackWich/Dock/src/Dock.Avalonia/Converters/ReferenceEqualsMultiConverter.cs
• Updated docs:
  • /Volumes/SSD/repos/StackWich/Dock/README.md
  • /Volumes/SSD/repos/StackWich/Dock/docfx/articles/dock-theme-browser.md
  • /Volumes/SSD/repos/StackWich/Dock/docfx/articles/dock-theme-fluent.md
  • /Volumes/SSD/repos/StackWich/Dock/docfx/articles/dock-theme-simple.md

Why

Tab switching performance regressed when content was recreated on each activation. This adds an explicit startup-time option to keep document views alive and restore fast tab switching behavior.

Usage

<browserTheme:BrowserTabTheme CacheDocumentTabContent="True" />

<dockFluent:DockFluentTheme CacheDocumentTabContent="True" />

<dockSimple:DockSimpleTheme CacheDocumentTabContent="True" />

Notes

• CacheDocumentTabContent is intended as a configuration-time choice (not a runtime toggle).
• Cached mode avoids ControlRecyclingDataTemplate for document tab content to prevent recycling-related behavior when content is intentionally kept alive.

Validation

• Built successfully with:
  • dotnet build /Volumes/SSD/repos/StackWich/Startups/Sandwich.Desktop/Sandwich.Desktop.csproj