Add RC.4 tooling support

[ObjectIsAdvantag]

Summary

• import the immutable MCP Description 0.8.0-rc.4 validator snapshot and frozen fixtures
• prepare @mcpdesc/validator 0.11.0 with active RC.3/RC.4 validation support
• prepare @mcpdesc/core 0.10.0 with RC.4 projection, merge, selection, and migration support
• retire RC.2 from current package APIs and document RC.4 component resolution as unsupported because snapshot provenance is unavailable

Validation

• npm run check
• npm run release:assess -- --json

AI assistance

GitHub Copilot assisted with implementation, tests, documentation, and release preparation. The maintainer reviewed the changes and remains accountable for the result.