New workspace package that owns built-in node bundles, one folder per
kind. Today the plugin is empty — no behavior change. Future commits
will add column/, shelf/, wall/, etc. and append each definition to
builtinPlugin.nodes.
The package depends on core (registry types) at v1; viewer, editor,
react, three are declared as peer deps so future node bundles can use
them without bumping their own version on every monorepo bump.
External plugins land as separate packages with the exact same shape
— this package is the dogfooded reference.
Tests: builtinPlugin shape + loadPlugin succeeds with zero kinds.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
