feat(plugin): persist XDATA to DWG and let plugins modify/delete entities (#249, #250)

#249 — plugin XDATA written via write_record survived only in memory: the
acadrust DWG writer dropped ExtendedData::records on save. Bump acadrust to
e88a9a6 (records now encode to EED and decode back on read) and fix the host
side that fed it:
  - ensure_app_id allocates a real APPID handle; a null handle serializes as
    0 and the EED reference can't resolve, so the XDATA vanished on reopen.
  - write_record / remove_record drop stale raw_dwg_eed for the target app so
    an edit made after a save/reopen wins over the pre-edit bytes.

#250 — out-of-process plugins got a throwaway document_mut() snapshot, so
edits to existing entities were silently discarded and deletion wasn't
expressible at all. Add the missing mutation surface:
  - UpdateEntity / RemoveEntity IPC requests + HostApi::update_entity /
    remove_entity (default in-process impls, RPC overrides on the client that
    invalidate the stale document cache).
  - Scene::update_entity replaces the entity in place, preserving its handle
    and owning block, and reseeds only its derived caches; remove reuses the
    cache-coherent erase_entities (which also honours layer locks).
  - document_mut() is documented as a local-only snapshot out-of-process.

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

crates/ocs_plugin_api/src/ipc/server.rs

@@ -28,6 +28,8 @@ pub fn handle_plugin_request(
             PluginResponse::Ok
         }
         AddEntity(entity) => PluginResponse::Handle(host.add_entity(entity)),
+        UpdateEntity(entity) => PluginResponse::Bool(host.update_entity(entity)),
+        RemoveEntity { handle } => PluginResponse::Bool(host.remove_entity(handle)),
         BumpGeometry => {
             host.bump_geometry();
             PluginResponse::Ok