feat(plugin): reference-only entity designation on the HostApi (API v9)

The HostApi gains set_reference_only(handle, reference_only): a plugin can
designate display geometry it owns and refreshes. The entity renders like any
other, but the built-in edit paths treat it like an entity on a locked layer,
while the owning plugin keeps full write access (update, erase, XDATA).

- Appended at the trait tail with a refusing default so plugins keep compiling
  against older hosts; the loader gate range is untouched.
- IPC: SetReferenceOnly / SetReferenceOnlyResult round-trip through the
  shared dispatch, the V3 client and the V4 client, with the shared document
  cache invalidated on success.
- build.rs version_info and manifest API_VERSION bumped to 9 in lockstep.
- Evidence: reference_only_request_and_response_roundtrip (bincode round-trip
  of the request and both response payloads).

crates/ocs_plugin_api/src/manifest.rs

@@ -24,8 +24,11 @@
 /// undoable replacement of a set of existing entities. v8 appends name-addressed
 /// XRecord object CRUD under named-object-dictionary paths
 /// (`HostApi::create_xrecord` / `read_xrecord` / `update_xrecord` /
-/// `delete_xrecord` / `list_xrecords`).
-pub const API_VERSION: u32 = 8;
+/// `delete_xrecord` / `list_xrecords`). v9 appends the reference-only entity
+/// designation (`HostApi::set_reference_only`): display-only geometry a plugin
+/// still owns and refreshes while the built-in edit paths treat it like a
+/// locked layer.
+pub const API_VERSION: u32 = 9;
 
 /// Oldest plugin API major the current host still loads. This keeps previously
 /// compiled cdylibs usable as long as their vtable layout is a prefix of the