Support external document tab strips in Dock drag/drop workflows

[danwalmsley]

PR Summary

This PR adds Dock support for hosting a DocumentTabStrip outside the DocumentControl/DockControl visual tree while preserving document tab drag/drop behavior, finalized with an explicit contract-based API.

What this fixes

• Document tabs can be dragged out from an external tab strip.
• Dock/host-window drag hover detects external tab strip drop areas.
• Dock adorner can appear over external tab strip targets.
• Drops work across the full tab strip surface (not only existing tab items).
• Tab drag preview can match the dragged tab size for a more consistent drag illusion.

Key changes

• Introduced an explicit external-surface contract:
  • IExternalDockSurface (DockControl owner + SurfaceControl).
• Added registration API on DockControl:
  • RegisterExternalDockSurface(...)
  • UnregisterExternalDockSurface(...)
  • internal weak-reference tracking, owner handoff between dock controls, and attach/detach lifecycle handling.
• Removed the legacy attached-property path:
  • deleted DockProperties.ExternalDockControl and related accessors/usage.
• Updated DocumentTabStrip to implement IExternalDockSurface.
• Updated DocumentTabStripItem drag start resolution:
  • now resolves owning dock control via DockHelpers.ResolveDockControl(...) (ancestor first, then registered external owner).
• Extended external hit-testing/drop discovery:
  • searches registered external surfaces for a dock control,
  • prefers dock-target hits for adorner activation,
  • falls back to external tab strip root so empty/trailing strip regions are droppable.
• Kept preferred drag-preview sizing for document tab drags (preview can match dragged tab dimensions).
• Added headless coverage for registration/ownership/hit-testing lifecycle:
  • tests/Dock.Avalonia.HeadlessTests/ExternalDockSurfaceTests.cs.
• Added sample app:
  • samples/DockExternalTabStripsSample
  • demonstrates two DockControls, each rooted by a DocumentDock, each with one registered external DocumentTabStrip, using stock Fluent tab visuals.

Commit breakdown

• Support external tab strip drop target discovery
• Allow external document tab strips to start dock drags
• Match tab drag preview size to dragged tab
• Introduce external dock surface contract and registration API
• Add headless tests for external dock surface registration
• Add external tab strips sample with two dock controls
• Follow-up polish/fixes:
  • use layout rounding for the tabstrip separators.
  • fix tests

Notes

• Existing in-tree DocumentTabStrip behavior remains unchanged.
• External usage now goes through explicit contract + registration API (legacy attached-property path removed).