Add demo_plugin and plugin host integration tests

Minimal in-tree add-on (DP_HELLO) validates registry, ribbon merge, and try_dispatch. Five new lib tests; EntryKind gains PartialEq for test helpers.

src/modules/mod.rs

@@ -139,6 +139,7 @@ pub mod home;
 pub mod insert;
 pub mod model;
 pub mod layout;
+pub mod demo_plugin;
 pub mod manage;
 pub mod view;