Plugins

See docs/guide/plugins.md for philosophy.

Interface in src/core/plugin/index.ts (new):

• Internal: src/core/equation (example)
• External: registerPlugin(def) validates schema, version, migrate, provides renderWeb, renderPdf/Odt/Docx, commands.

All plugins declare type, version, schema, normalize, commands, formatters, migrate, and are tested via contract tests (tests/plugin/).

See src/public-api/index.ts (registerPlugin) and docs/guide/plugins.md.