[ui] Add semantic primitive ownership Build a layered Zenbu token and sizing system, make authored controls use native-underneath primitives, migrate mrjunejune without imposing visual surfaces, and document/enforce HTML ownership in the catalog and wiki. Co-authored-by: Copilot <[email protected]>

#ifndef UVBOOK_PLUGIN_SYSTEM
#define UVBOOK_PLUGIN_SYSTEM

// Plugin authors should use this to register their plugins with mfp.
void mfp_register(const char *name);

#endif