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/mod.rs

@@ -297,6 +297,9 @@ pub(super) struct OpenCADStudio {
     /// keep their manifest listed but drop their ribbon tab and command
     /// dispatch. Persisted via [`settings::UserSettings::disabled_plugins`].
     disabled_plugins: rustc_hash::FxHashSet<String>,
+    /// External add-on packages found in the plugins folder, refreshed when the
+    /// Plugin Manager opens. Discovery only — not yet loaded (phase 2).
+    external_plugins: Vec<crate::plugin::external::ExternalPlugin>,
     /// PDSIZE text buffer for the Point Style (DDPTYPE) dialog.
     point_size_buf: String,
     /// Point Style size mode: `true` = relative to screen, `false` = absolute.
@@ -1461,6 +1464,7 @@ impl OpenCADStudio {
             modal_drag_last: None,
             modal_dragging: false,
             disabled_plugins: rustc_hash::FxHashSet::default(),
+            external_plugins: Vec::new(),
             point_size_buf: String::new(),
             point_size_relative: true,
             default_assoc_prompted: false,

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()
             }

src/app/view.rs

@@ -1124,6 +1124,7 @@ impl OpenCADStudio {
                 crate::ui::plugin_manager::view_window(
                     &crate::plugin::installed_manifests(),
                     &self.disabled_plugins,
+                    &self.external_plugins,
                 ),
                 520,
                 460,