Adopt Changesets v3 for viewer packages - #13

Summary

• adopt Changesets CLI v3 for the eight public @baseblocks/anydoc-* packages
• keep the viewer packages in one fixed alpha prerelease group while excluding the cross-language @firecrawl/anydoc release family
• require release intent in credentialless pull-request CI
• validate tagged publish plans while preserving the existing build, checksum, registry-integrity, and viewer-v* publication gates
• document the feature, version-PR, and publishing workflows

Validation

• full viewer build, typecheck, and test suite
• viewer and ingestion budgets
• package dry-runs and packed server/browser closure checks
• Changesets metadata/status and workflow linting
• synthetic patch changeset verified all eight packages and internal ranges advance from 0.1.0-alpha.13 to 0.1.0-alpha.14