[codex] Fix release markdown rendering

[Xueyang-Song]

Summary

Fixes release-mode markdown previews rendering only the first visible block by avoiding an unconditional CanvasTextLayout.LineMetrics read during decoration drawing.

Root Cause

In Release/AOT, the WinRT projection can throw NotSupportedException while marshalling CanvasLineMetrics. The renderer fetched line metrics unconditionally in the inline decoration pass, even when the block did not need underline or strikethrough decoration. That exception aborted the tile paint after the first block, leaving headings and non-virtualized overlay controls visible while the rest of the markdown body was blank.

Changes

• Lazily fetch line metrics only when a run actually needs decoration.
• Fall back to character-region bounds when line metrics cannot be marshalled, so text keeps painting in Release.
• Measure lazy layout against the CanvasVirtualControl paint region so requested tiles are realized before drawing.
• Add an unpackaged release fallback for repo file cache storage so the published app can open repo-code markdown previews in this test path.

Validation

• dotnet test MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -c Debug -p:Platform=x64 --nologo passed: 292 tests.
• dotnet build JitHub.WinUI\JitHub.WinUI.csproj -c Release -p:Platform=x64 --nologo succeeded.
• dotnet publish JitHub.WinUI\JitHub.WinUI.csproj -c Release -p:Platform=x64 --no-restore --nologo succeeded with existing trim/AOT warnings.
• Published Release app opened JitHubApp/JitHubV2 repo code page and rendered README.md in Rich mode with multiple markdown blocks visible.