Add opt-in OpenAPI primary-spec mode

[danielporterda]

Summary

• add opt-in --primary-spec-id support for OpenAPI multi-spec outputs
• add opt-in --link-prefix support for route-stable overview and additional-spec links
• keep existing OpenAPI output unchanged unless the new flags are supplied

Testing

• direnv exec /Users/danielporter/control/.worktrees/x2mdx-openapi-primary-spec-scan bash -lc 'PYTHONPATH=src python -m unittest tests.test_openapi'
• direnv exec /Users/danielporter/control/.worktrees/x2mdx-openapi-primary-spec-scan bash -lc 'PYTHONPATH=src pytest tests/test_openapi.py -q'