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/client.rs

@@ -692,6 +692,23 @@ impl HostApi for PluginHostApi {
             Err(error) => Err(error.to_string()),
         }
     }
+
+    fn set_reference_only(&mut self, handle: Handle, reference_only: bool) -> Result<(), String> {
+        match self.client.request(PluginRequest::SetReferenceOnly {
+            handle,
+            reference_only,
+        }) {
+            Ok(PluginResponse::SetReferenceOnlyResult(result)) => {
+                if result.is_ok() {
+                    self.document_cache = OnceCell::new();
+                }
+                result
+            }
+            Ok(PluginResponse::Error(error)) => Err(error),
+            Ok(other) => Err(format!("unexpected reference-only response: {other:?}")),
+            Err(error) => Err(error.to_string()),
+        }
+    }
 }
 
 /// Sentinel reader used when the shared-memory view could not be initialized.