feat(plugin): load external cdylib plugins at runtime (phase 2)

Move BuiltinPlugin into ocs_plugin_api (host feature) so out-of-tree
crates can implement it, and add export_plugin! to emit the two C
symbols a cdylib exposes: ocs_plugin_api_version (checked first, so an
ABI-incompatible build never runs) and ocs_plugin_register -> boxed
BuiltinPlugin. The host loads every compatible package from the plugins
folder at startup via libloading (desktop only), keeps the library
resident for the session, and merges its ribbon tab + command dispatch
into the same paths as built-ins (honouring enable/disable). The Plugin
Manager shows external packages with a Loaded / incompatible status.

Approach B: the plugin hands back a boxed trait object, assuming a
matching toolchain + ocs_plugin_api version (the version symbol enforces
the latter). crates/ocs_example_plugin is the reference cdylib.

Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/ui/plugin_manager.rs

@@ -156,13 +156,15 @@ fn status_badge<'a>(label: &str, color: Color) -> Element<'a, Message> {
         .into()
 }
 
-fn external_card<'a>(p: &ExternalPlugin) -> Element<'a, Message> {
-    let (status, color) = if !p.api_compatible() {
+fn external_card<'a>(p: &ExternalPlugin, loaded: bool) -> Element<'a, Message> {
+    let (status, color) = if loaded {
+        ("Loaded", Color { r: 0.2, g: 0.5, b: 0.3, a: 1.0 })
+    } else if !p.api_compatible() {
         ("API incompatible", Color { r: 0.55, g: 0.28, b: 0.28, a: 1.0 })
     } else if !p.lib_present {
         ("No library", Color { r: 0.5, g: 0.42, b: 0.2, a: 1.0 })
     } else {
-        ("Ready to load", Color { r: 0.2, g: 0.45, b: 0.28, a: 1.0 })
+        ("Restart to load", Color { r: 0.5, g: 0.42, b: 0.2, a: 1.0 })
     };
     let header = row![
         text(p.name.clone()).size(15).color(WHITE),
@@ -201,6 +203,7 @@ pub fn view_window<'a>(
     plugins: &[&'static PluginManifest],
     disabled: &FxHashSet<String>,
     externals: &[ExternalPlugin],
+    loaded: &FxHashSet<String>,
 ) -> Element<'a, Message> {
     let title = text("Installed Plugins").size(20).color(WHITE);
     let subtitle = text(format!(
@@ -229,7 +232,7 @@ pub fn view_window<'a>(
                 .color(DIM),
         );
         for p in externals {
-            list = list.push(external_card(p));
+            list = list.push(external_card(p, loaded.contains(&p.id)));
         }
     }
     let body: Element<'_, Message> = scrollable(list.width(Fill)).height(Fill).into();