LT-22691: Publish writing system under cursor

[mark-sil]

The detail editor focus hook now also publishes the window-scoped WritingSystemUnderCursorChanged event, so the toolbar writing-system combo tracks the cursor while the Avalonia detail view is focused.

Notes:

• This adds the first Avalonia-to-WinForms state channel rather than replacing an adapter route.
• The view publishes an event instead of writing the property itself so the relevant code stays quarantined in the one listener that already owns that combo. The view stays ignorant of its consumers, and a replacement toolbar later subscribes to the same event without touching the view.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   10m 14s ⏱️ +26s
5 834 tests +1  5 753 ✅ +1  81 💤 ±0  0 ❌ ±0 
5 843 runs  +1  5 762 ✅ +1  81 💤 ±0  0 ❌ ±0 

Results for commit e258238. ± Comparison against base commit 70076f6.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.33%. Comparing base (caeeeb1) to head (e258238).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...xWorks/Avalonia/Hosting/RecordEditView.Avalonia.cs	66.66%	5 Missing and 1 partial ⚠️
Src/xWorks/TextListeners.cs	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1095      +/-   ##
==========================================
+ Coverage   38.31%   38.33%   +0.01%     
==========================================
  Files        1507     1507              
  Lines      350524   350554      +30     
  Branches    40288    40291       +3     
==========================================
+ Hits       134302   134370      +68     
+ Misses     186990   186954      -36     
+ Partials    29232    29230       -2     

Files with missing lines	Coverage Δ
Src/Common/SimpleRootSite/SimpleRootSite.cs	55.09% <100.00%> (-0.30%)	⬇️
Src/xWorks/TextListeners.cs	27.50% <80.00%> (+27.50%)	⬆️
...xWorks/Avalonia/Hosting/RecordEditView.Avalonia.cs	45.93% <66.66%> (+0.84%)	⬆️

... and 6 files with indirect coverage changes

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[jasonleenaylor]

@jasonleenaylor reviewed 5 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on mark-sil).