Summary

• create llm.zip through the exporter-provided IFileSystem instead of physical ZipFile APIs
• enumerate and copy Markdown files through IFileSystem so docs-builder serve works with MockFileSystem output
• create the output directory when the exporter is finalized without prior page output
• add a regression test that builds and inspects the archive entirely in memory

Validation

• ./build.sh lint
• dotnet test tests/Elastic.Markdown.Tests/ --no-restore --disable-build-servers -m:1

All 1,895 Elastic.Markdown tests pass.