[codex] Improve markdown accessibility and selection

[Xueyang-Song]

Summary

This PR brings the WinUI markdown renderer much closer to native-control accessibility behavior and hardens the selection/clipboard path that shook out during manual testing.

What Changed

• Added a semantic accessibility model and richer UIA peer tree for headings, paragraphs, links, images, lists, tables/cells, code blocks, and hosted embeds.
• Implemented TextPattern support with document/visible ranges, movement, selection, bounding rectangles, range-from-point behavior, scroll-into-view, and word-sized Narrator rectangles.
• Added high contrast/system theme integration, system FlowDirection helper support, localized automation/context strings, and native-looking focus/keyboard coordination across painted links and hosted WinUI controls.
• Reworked text selection rendering onto a single stable Win2D adorner so selection uses native colors without repainting/shaking document text.
• Fixed selection edge cases for thematic breaks, cross-page persistence, multi-control dismissal, table row-border hit-testing, and Ctrl+C after pointer selection.
• Expanded sample automation and unit coverage, and updated markdown renderer docs/TODOs.

Root Causes Fixed

• Narrator only saw a custom/root-like surface because text ranges and semantic peers were too flat.
• Selection color/shake regressions came from painting selection in the document text layer or covering text without repainting selected foreground.
• Table drag flicker came from table-gap hit testing returning the table container block index instead of a nearby real cell caret.
• Ctrl+C after drag could miss the renderer because WinUI focus routing may remain on a stale element after pointer selection.

Validation

• dotnet build MarkdownRenderer\MarkdownRenderer.Sample\MarkdownRenderer.Sample.csproj -p:Platform=x64 --no-restore
• dotnet build MarkdownRenderer\MarkdownRenderer.Sample.Automation\MarkdownRenderer.Sample.Automation.csproj -p:Platform=x64 --no-restore
• dotnet test MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -p:Platform=x64 --no-restore -> 218 passed
• Full sample automation run -> 26 passed, 0 failed
• git diff --cached --check

Notes

Narrator screenshot artifacts generated during manual smoke testing are intentionally not included in this PR.