[codex] Prepare MarkdownRenderer for 1.0 readiness

[Xueyang-Song]

Summary

• Finish the MarkdownRenderer 1.0 readiness pass across rendering, theming, selection, performance, packaging, public API, and documentation.
• Add GFM/Markdown Extra helpers, document-query facade APIs, copy options, quick-start builders, and release-ready docs-site structure.
• Harden inline/block images, selected-image visuals, graphics-device-loss handling, lazy layout, embed selection, auto-scroll selection, and native SVG asset packaging.
• Ship ThorVG native assets for x86, x64, and ARM64, with tests validating architecture assets.

User/developer impact

• Apps get richer markdown support and more predictable native behavior without changing the default source-markdown clipboard contract.
• Consumers have a clearer public API surface through builder helpers and MarkdownRenderer.Document.MarkdownDocument queries.
• Documentation now reads like a hosted docs site: feature matrix, API guide, troubleshooting, samples, packaging, and release checklist.

Validation

• git diff --cached --check
• Markdown docs link/typo sanity scan
• dotnet test MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -c Debug -p:Platform=x64 (252 passed, existing analyzer warnings only)

Notes

• Raw HTML and LaTeX/math remain explicitly out of scope for this PR and are documented as deferred tracks.
• Manual release smoke is still called out in the docs for Narrator, contrast themes, graphics reset, clipboard apps, and x86/x64/ARM64 sample launch.