Complete MarkdownRenderer 1.0 native feature set

[Xueyang-Song]

Summary

• complete the modular MarkdownRenderer 1.0 architecture and public API surface, including profiles and extension composition
• add safe HTML, native TeX math, native Mermaid/MMIR diagrams, TextMate syntax grammars, ThorVG/SVG, and the GitHub rendering profile
• complete mouse, keyboard, touch, and UI Automation selection, native task-list checkboxes, context commands, scrolling, virtualization, localization, theming, and High Contrast behavior
• rebuild the sample app around scalable side navigation with dedicated Math, Mermaid, HTML, task-list, image, and touch-selection examples
• add conformance, performance, accessibility, AOT, package, provenance, security, and release-evidence gates

Validation

• dotnet restore JitHub.slnx -p:RestorePackagesWithLockFile=true -p:RestoreLockedMode=false -v minimal
• dotnet build JitHub.slnx -c Debug --no-restore -v minimal -tl:off — 0 warnings, 0 errors
• MarkdownRenderer.GitHub.Tests — 277 passed
• MarkdownRenderer.Math.Tests — 75 passed
• MarkdownRenderer.Mermaid.Tests — 103 passed
• timeout-sensitive orchestration preflight — passed serially
• both scroll-trial-ui-p95 external-gate mutation cases — passed serially
• Math sample — 27 native formulas and 7 display vector scenes, with no diagnostics across Light, Dark, 200% scaling, and forced High Contrast
• Mermaid sample — 17 native diagrams with no fallbacks across Light, Dark, 200% scaling, forced High Contrast, pipeline, sequence, and pie scenarios

Notes

The broad core test run exposed two subprocess timeouts only while several performance-heavy suites were running concurrently. Both affected paths passed when rerun serially; focused feature suites and the full solution build are green.