feat(plugin): discover external plugin packages from the plugins folder

Phase-2 groundwork: scan <config>/OpenCADStudio/plugins/<id>/plugin.toml,
parse the documented keys (no new TOML dep), check for a platform native
library beside it, and gate on the API version. The Plugin Manager gains
an "External (plugins folder)" section showing each package with a
status pill (Ready to load / API incompatible / No library). Discovery
only — actually loading the cdylib is the next step.

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

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

src/app/update.rs

@@ -5526,6 +5526,9 @@ impl OpenCADStudio {
 
             // ── Plugin Manager window ─────────────────────────────────────
             Message::PluginManagerOpen => {
+                // Refresh the on-disk external-plugin list each time the manager
+                // opens so newly dropped-in packages show up.
+                self.external_plugins = crate::plugin::external::discover();
                 self.active_modal = Some(super::ModalKind::PluginManager);
                 Task::none()
             }