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/protocol.rs

@@ -226,6 +226,11 @@ pub enum PluginRequest {
     ListXRecords {
         dictionary_path: Vec<String>,
     },
+    /// V9 (additive): mark an entity reference-only (plugin-owned display data).
+    SetReferenceOnly {
+        handle: Handle,
+        reference_only: bool,
+    },
 }
 
 /// Responses the host sends back for `PluginRequest`.
@@ -272,6 +277,8 @@ pub enum PluginResponse {
     XRecordDeleteResult(Result<crate::host::XRecord, String>),
     /// V8 (additive): XRecord names under a dictionary path.
     XRecordNames(Result<Vec<String>, String>),
+    /// V9 (additive): outcome of the reference-only designation.
+    SetReferenceOnlyResult(Result<(), String>),
 }
 
 /// Messages sent from the host to the plugin runner.