Publish complete API documentation

[Stephanvs]

What changed

• documents the complete public API surface with XML comments
• enables XML documentation generation without suppressing CS1591
• explicitly disables MSTest parallelization because serialization tests change process-wide culture
• publishes a final README covering the ordered model, typed features, multi-target package, safety model, CLI boundary, and roadmap
• records only the intentional v2 API breaks in CompatibilitySuppressions.xml, while keeping baseline package validation active for any additional breaks

Validation

• combined Release build across netstandard2.0, net8.0, and net10.0: 0 warnings, 0 errors
• combined tests: 33/33 passed, including real D2 parser validation
• generated XML documentation: valid and included for all three package assets
• combined package validation against 1.0.0: passed with the checked-in intentional-v2 suppressions
• .nupkg and .snupkg creation and content inspection: passed

Stack and merge order

Merge independent PR #6 first or at any point. Then merge #4; after that, #5 and #7 can be retargeted to main and merged in either order. Merge #8 next and retarget this PR to main last. Release the resulting API as 2.0.0 because the compatibility suppressions document intentional major-version changes.