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/mod.rs

@@ -0,0 +1,47 @@
+// Minimal in-tree add-on — validates plugin host on `feature/plugin-host`.
+
+pub mod dispatch;
+pub mod manifest;
+pub mod plugin;
+pub mod register;
+
+use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+inventory::submit!(crate::command::CommandRegistration {
+    names: &["DP_HELLO"]
+});
+
+pub struct DemoPluginModule;
+
+impl CadModule for DemoPluginModule {
+    fn id(&self) -> &'static str {
+        "demo_plugin"
+    }
+
+    fn title(&self) -> &'static str {
+        "Demo Plugin"
+    }
+
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![RibbonGroup {
+            title: "Smoke",
+            tools: vec![RibbonItem::LargeTool(ToolDef {
+                id: "DP_HELLO",
+                label: "Hello",
+                icon: IconKind::Glyph("★"),
+                event: ModuleEvent::Command("DP_HELLO".to_string()),
+            })],
+        }]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::plugin::all_ribbon_modules;
+
+    #[test]
+    fn ribbon_tab_is_registered() {
+        let titles: Vec<&str> = all_ribbon_modules().iter().map(|m| m.title()).collect();
+        assert!(titles.contains(&"Demo Plugin"), "tabs: {titles:?}");
+    }
+}