Release 0.13.0-alpha.3 structured data and image exports

[MiLattanzio]

Summary

• add versioned deterministic JSON, XML, and XHTML document/page exports
• retain normalized/raw PDF font names and stable text, font, link, and image IDs
• export reusable JPEG, JP2, and JBIG2 payloads with bounded PNG fallbacks
• add structured bundles, CLI commands, WebAssembly downloads, schemas, and documentation
• update version, release, package, and tool metadata to 0.13.0-alpha.3

Validation

• Release solution build: 0 warnings/errors
• NUnitLite: 315/315
• managed-only verifier
• library/tool pack and package verification
• clean net8/net10 consumers
• packaged dotnet tool HTML, separate, JSON, and export smoke tests
• Blazor WebAssembly Release build

Closes #30