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>

Cargo.lock

@@ -71,7 +71,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#4ae2ceb72f00b69e983b20111c809aecf138814c"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#e88a9a6afdcf20db4c1f3140088b2f4b8b851e2b"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",