Merge plugins into server-lua-api

The new services and throwdown kit API files use add_functions and the shared
helpers; add_functions opens a read-only parent for reskate.throwdowns.flag.
throwdowns.release_flag stays removed, as plugins has it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Extension/Plugins/plugin_api_ui.cpp

@@ -502,7 +502,7 @@ struct UiApi {
             return 1;
         }
         if (!plugin.services_.ui_sound(name))
-            luaL_error(L, "there is no menu sound \"%s\": select, cancel, navigate, toggle or tab", name.c_str());
+            luaL_error(L, "there is no menu sound \"%s\": select, cancel, navigate, toggle, tab, countdown or go", name.c_str());
         lua_pushboolean(L, 1);
         return 1;
     }