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/build.rs

@@ -1263,7 +1263,7 @@ fn generate_version_info(out_dir: &Path) {
         "acadrust_version": opencadcodec.version.to_string(),
         "acadrust_source": opencadcodec.source.as_ref().map(|s| s.to_string()),
         "rustc_version": rustc_version,
-        "api_version": 8,
+        "api_version": 9,
         "api_version_min_supported": 3,
         "build_timestamp": build_timestamp,
     });