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/ipc/server.rs

@@ -151,5 +151,11 @@ pub fn handle_plugin_request(
             let path: Vec<&str> = dictionary_path.iter().map(String::as_str).collect();
             PluginResponse::XRecordNames(host.list_xrecords(&path))
         }
+        SetReferenceOnly {
+            handle,
+            reference_only,
+        } => {
+            PluginResponse::SetReferenceOnlyResult(host.set_reference_only(handle, reference_only))
+        }
     }
 }