Release lean RC.2 and RC.3 support

[ObjectIsAdvantag]

Summary

• narrow active validator and core support to RC.2 and RC.3
• mark RC.2 deprecated and remove retired selectors from current APIs, bundles, and tarballs
• retain historical snapshot sources and fixtures in Git while removing the retained intake manifest
• move core release history into its package and make the root changelog a package index
• prepare @mcpdesc/validator@0.10.1 and @mcpdesc/core@0.9.1

Validation

• npm run check
• npm run release:assess -- --json
• validator: 229 tests, declarations, CSP bundles, and package tarball
• core: 52 tests, browser bundles, package tarball, and isolated consumer

Release order

1. Publish and verify @mcpdesc/validator@0.10.1.
2. Re-run core readiness against the published validator.
3. Publish and verify @mcpdesc/core@0.9.1.

AI assistance

GitHub Copilot implemented the support-window change, updated tests and documentation, prepared release metadata, and ran validation. The maintainer reviewed the scope and approved both versions and publication. The maintainer remains accountable for the result.