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/dispatch.rs

@@ -0,0 +1,11 @@
+use crate::plugin::host::HostSession;
+
+pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+    match cmd {
+        "DP_HELLO" => {
+            host.push_info("Hello from demo_plugin (plugin host OK).");
+            true
+        }
+        _ => false,
+    }
+}

src/modules/demo_plugin/manifest.rs

@@ -0,0 +1,14 @@
+use crate::plugin::manifest::{ApiVersion, PluginManifest};
+
+pub const PLUGIN_ID: &str = "opencad.demo_plugin";
+
+pub static MANIFEST: PluginManifest = PluginManifest {
+    id: PLUGIN_ID,
+    name: "Demo Plugin",
+    version: "0.1.0",
+    description: "Minimal add-on for plugin-host integration tests",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 99,
+    xdata_apps: &[],
+    command_prefixes: &["DP_"],
+};

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:?}");
+    }
+}

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)
+    }
+}

src/modules/demo_plugin/plugin.toml

@@ -0,0 +1,13 @@
+[plugin]
+id = "opencad.demo_plugin"
+name = "Demo Plugin"
+version = "0.1.0"
+description = "Minimal add-on for plugin-host integration tests"
+author = "Open CAD Studio contributors"
+license = "GPL-3.0-only"
+
+[opencad]
+api_version = 1
+ribbon_order = 99
+command_prefixes = ["DP_"]
+xdata_apps = []

src/modules/demo_plugin/register.rs

@@ -0,0 +1,7 @@
+use super::plugin::DemoPlugin;
+
+inventory::submit! {
+    crate::plugin::registry::PluginRegistration {
+        construct: || Box::new(DemoPlugin),
+    }
+}

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;