refactor(plugin): drop built-in plugins, external-only

OpenCADStudio no longer bundles plugins — every add-on is an external
cdylib loaded from the plugins folder or installed via the marketplace.

- Remove the demo plugin and the in-tree ocs_example_plugin crate.
- Remove the inventory-based built-in registration path (PluginRegistration,
  all_plugins, installed_manifests); registry now serves core ribbon tabs
  plus loaded external plugins, and try_dispatch routes only to externals.
- Plugin Manager drops the 'compiled into this build' section; the
  enable/disable toggle moves onto loaded external cards.
- Prune the now-dead HostSession/DocumentTab helpers and re-exports.

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

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

src/app/document.rs

@@ -9,7 +9,7 @@ use crate::ui::{LayerPanel, PropertiesPanel};
 use acadrust::tables::Ucs;
 use acadrust::{CadDocument, Handle};
 use iced;
-use std::any::{Any, TypeId};
+use std::any::Any;
 use std::collections::HashMap;
 use std::path::PathBuf;
 
@@ -216,37 +216,6 @@ impl DocumentTab {
         }
     }
 
-    pub(super) fn plugin_state<T: Any + Send + Sync + 'static>(
-        &self,
-        plugin_id: &'static str,
-        _type_id: TypeId,
-    ) -> Option<&T> {
-        self.plugin_state.get(plugin_id)?.downcast_ref::<T>()
-    }
-
-    pub(super) fn plugin_state_mut<T: Any + Send + Sync + 'static>(
-        &mut self,
-        plugin_id: &'static str,
-        _type_id: TypeId,
-    ) -> Option<&mut T> {
-        self.plugin_state.get_mut(plugin_id)?.downcast_mut::<T>()
-    }
-
-    pub(super) fn ensure_plugin_state<T: Any + Send + Sync + 'static>(
-        &mut self,
-        plugin_id: &'static str,
-        init: impl FnOnce() -> T,
-    ) -> &mut T {
-        if !self.plugin_state.contains_key(plugin_id) {
-            self.plugin_state.insert(plugin_id, Box::new(init()));
-        }
-        self.plugin_state
-            .get_mut(plugin_id)
-            .expect("just inserted")
-            .downcast_mut::<T>()
-            .expect("plugin_id type mismatch")
-    }
-
     /// Welcome / Start tab. Carries a dummy Scene so the rest of the app
     /// can read tab state uniformly; the viewport renderer detects
     /// `is_start` and shows a welcome page instead.