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 <noreply@anthropic.com>

crates/ocs_example_plugin/src/lib.rs

@@ -0,0 +1,68 @@
+//! Reference external add-on, built as a `cdylib` the host loads at runtime.
+//!
+//! It depends only on `ocs_plugin_api` (with the `host` feature) — never on the
+//! `OpenCADStudio` binary — so it demonstrates the stable contract an
+//! out-of-tree plugin targets: a `PluginManifest`, a `CadModule` ribbon tab, a
+//! `BuiltinPlugin` entry point, and the `export_plugin!` C-ABI export.
+
+use ocs_plugin_api::host::{BuiltinPlugin, HostApi};
+use ocs_plugin_api::manifest::{ApiVersion, PluginManifest};
+use ocs_plugin_api::ribbon::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+static MANIFEST: PluginManifest = PluginManifest {
+    id: "opencad.example",
+    name: "Example Plugin",
+    version: "0.1.0",
+    description: "Reference dynamically-loaded add-on",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 50,
+    xdata_apps: &[],
+    command_prefixes: &["EX_"],
+};
+
+/// Ribbon tab for the example plugin.
+struct ExampleModule;
+
+impl CadModule for ExampleModule {
+    fn id(&self) -> &'static str {
+        "example"
+    }
+    fn title(&self) -> &'static str {
+        "Example"
+    }
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![RibbonGroup {
+            title: "Demo",
+            tools: vec![RibbonItem::LargeTool(ToolDef {
+                id: "EX_HELLO",
+                label: "Hello",
+                icon: IconKind::Glyph("◆"),
+                event: ModuleEvent::Command("EX_HELLO".to_string()),
+            })],
+        }]
+    }
+}
+
+/// The plugin entry point handed to the host.
+struct ExamplePlugin;
+
+impl BuiltinPlugin for ExamplePlugin {
+    fn manifest(&self) -> &'static PluginManifest {
+        &MANIFEST
+    }
+    fn ribbon(&self) -> Box<dyn CadModule> {
+        Box::new(ExampleModule)
+    }
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
+        match cmd {
+            "EX_HELLO" => {
+                host.push_info("Hello from the external example plugin (cdylib loaded).");
+                true
+            }
+            _ => false,
+        }
+    }
+}
+
+// Emit the C-ABI symbols the host loader looks for.
+ocs_plugin_api::export_plugin!(ExamplePlugin);