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/demo_plugin/plugin.rs

@@ -0,0 +1,21 @@
+use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::manifest::PluginManifest;
+
+use super::dispatch;
+use super::manifest;
+
+pub struct DemoPlugin;
+
+impl BuiltinPlugin for DemoPlugin {
+    fn manifest(&self) -> &'static PluginManifest {
+        &manifest::MANIFEST
+    }
+
+    fn ribbon(&self) -> Box<dyn crate::modules::CadModule> {
+        Box::new(super::DemoPluginModule)
+    }
+
+    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+        dispatch::handle(host, cmd)
+    }
+}