Add branded report rendering to FileContext

[KSemenenko]

Summary

• Add versioned FileContext 1.0.6 report generation with branded PDF, self-contained HTML, and chart PNG outputs.
• Support headings, formatted text, KPI rows, five chart types, tables, images, callouts, and page breaks with page headers and footers.
• Document the public API and add report rendering regressions.

Verification

• dotnet build ManagedCode.FileContext.slnx --configuration Release
• dotnet test tests/ManagedCode.FileContext.Tests/ManagedCode.FileContext.Tests.csproj --configuration Release /p:CollectCoverage=true /p:CoverletOutput=coverage /p:CoverletOutputFormat=opencover (143 passed; 95.68% line coverage)
• dotnet format ManagedCode.FileContext.slnx --verify-no-changes
• dotnet pack src/ManagedCode.FileContext/ManagedCode.FileContext.csproj --configuration Release --no-build --output artifacts
• Visually inspected the generated two-page sample PDF.