feat(plugin): enable/disable plugins from the Plugin Manager

The manager was a read-only inventory. Each plugin now has an
Enable/Disable toggle: a disabled plugin keeps its manifest listed but
drops its ribbon tab and command dispatch. The disabled-id set persists
in settings.txt (disabled_plugins=) and the ribbon rebuilds on toggle.

Part of the #100 extensibility epic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plugin/mod.rs

@@ -8,6 +8,6 @@ pub mod host;
 pub mod manifest;
 pub mod registry;
 
-pub use registry::all_ribbon_modules;
+pub use registry::{all_ribbon_modules, ribbon_modules_enabled};
 pub(crate) use host::BuiltinPlugin;
 pub(crate) use registry::{installed_manifests, try_dispatch};

src/plugin/registry.rs

@@ -28,11 +28,20 @@ pub fn installed_manifests() -> Vec<&'static PluginManifest> {
     manifests
 }
 
-/// Core ribbon tabs plus add-on tabs (sorted by `manifest.ribbon_order`).
+/// Core ribbon tabs plus *every* add-on tab (sorted by `manifest.ribbon_order`).
 pub fn all_ribbon_modules() -> Vec<Box<dyn CadModule>> {
+    ribbon_modules_enabled(&rustc_hash::FxHashSet::default())
+}
+
+/// Core ribbon tabs plus add-on tabs whose plugin id is **not** in `disabled`
+/// (sorted by `manifest.ribbon_order`). Used by the Plugin Manager toggle.
+pub fn ribbon_modules_enabled(
+    disabled: &rustc_hash::FxHashSet<String>,
+) -> Vec<Box<dyn CadModule>> {
     let mut core = core_registry::all_modules();
     let mut addons: Vec<(i32, Box<dyn CadModule>)> = all_plugins()
         .into_iter()
+        .filter(|p| !disabled.contains(p.manifest().id))
         .map(|p| (p.manifest().ribbon_order, p.ribbon()))
         .collect();
     addons.sort_by_key(|(order, _)| *order);
@@ -40,10 +49,15 @@ pub fn all_ribbon_modules() -> Vec<Box<dyn CadModule>> {
     core
 }
 
-/// Try each plugin until one handles `cmd`. Returns true if handled.
+/// Try each *enabled* plugin until one handles `cmd`. Returns true if handled.
+/// Disabled plugins (toggled off in the Plugin Manager) are skipped.
 pub(crate) fn try_dispatch(app: &mut OpenCADStudio, tab: usize, cmd: &str) -> bool {
+    let disabled = app.disabled_plugin_ids();
     let mut host = HostSession::new(app, tab);
     for plugin in all_plugins() {
+        if disabled.contains(plugin.manifest().id) {
+            continue;
+        }
         if plugin.dispatch(&mut host, cmd) {
             return true;
         }
@@ -98,6 +112,22 @@ mod tests {
         assert_eq!(titles.len(), core.len() + all_plugins().len());
     }
 
+    #[test]
+    fn disabled_plugin_drops_its_ribbon_tab() {
+        let mut disabled = rustc_hash::FxHashSet::default();
+        disabled.insert("opencad.demo_plugin".to_string());
+        let titles: Vec<&str> = ribbon_modules_enabled(&disabled)
+            .iter()
+            .map(|m| m.title())
+            .collect();
+        assert!(
+            !titles.contains(&"Demo Plugin"),
+            "disabled plugin still present: {titles:?}"
+        );
+        // Only the add-on tab is dropped; core tabs stay.
+        assert_eq!(titles.len(), core_registry::all_modules().len());
+    }
+
     #[test]
     fn try_dispatch_routes_demo_command() {
         let mut app = OpenCADStudio::new_for_test();