[codex] Fix packaged markdown text selection

[Xueyang-Song]

Summary

Fixes the packaged-build markdown/code-viewer selection crash and first-drag flash by keeping the preview renderer stable during queued view-model refreshes and hardening the markdown selection overlay lifecycle.

Root Cause

In packaged x86 builds, the first pointer interaction could trigger a queued preview refresh that replaced the markdown renderer while it owned pointer capture. That dropped capture mid-drag and, in the crash path, caused overlay children to be added during WinUI measure/arrange.

Changes

• Reuse the existing file preview renderer when the preview kind is unchanged.
• Pre-create and safely reattach the markdown selection adorner and drag shield.
• Ignore touch contacts in markdown selection/hover handling so touch scroll remains owned by the containing ScrollViewer.
• Require selection move/release/cancel events to match the active captured pointer.
• Draw selected glyph foreground with a masked text layout pass instead of per-selection-rect clipped layers.

Validation

• dotnet test MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -c Release -p:Platform=x64 --no-restore
• dotnet test JitHub.WinUI.Tests\JitHub.WinUI.Tests.csproj -c Release -p:Platform=x64
• dotnet build JitHub.WinUI\JitHub.WinUI.csproj -c Release -p:Platform=x86 -p:GenerateAppxPackageOnBuild=false --no-restore
• Registered and launched the x86 packaged-style build locally; user confirmed the crash/first-drag issue is fixed.