* feat(plugins): add React Native UI primitives
Give client plugins host-owned adaptive UI without exposing app internals. Preserve plugin runtime contexts when modal content moves through the compact portal host.
* docs(plugins): clarify host UI reference
* refactor(plugins): harden host UI boundary
Replace mocked pass-through coverage with a real browser plugin journey, keep host runtime ownership inside the evaluator, preserve toast context across compact portals, and expose modal content as a compound component.
* fix(plugins): defer native UI presentation runtime
Loading the plugin evaluator initialized the modal and toast presentation stacks even when no plugin requested host UI. Keep the capability stable while delaying platform-heavy presentation code until Modal.Content renders.
* refactor(plugins): move modal metadata to root
Keep Modal.Content body-only while the top-level Modal owns the host-rendered title and leading icon. This leaves presentation metadata stable as the compound API gains future regions.