Add Mintlify-parity reference renderer

[danielporterda]

Summary

• add a shared x2mdx reference renderer for Protobuf, OpenRPC, and AsyncAPI operation pages
• add Mintlify-like operation layouts, right-rail copyable examples, lifecycle/schema/example sections, and AsyncAPI multipage output
• add flake-based direnv/nix setup and CI coverage for reproducible test runs

Validation

• direnv exec /Users/danielporter/control/.worktrees/x2mdx-mintlify-parity-renderer pytest
• representative generated MDX compile checks in downstream docs
• local path scan for workstation-only references