feat(plugin): harden V4 host integration

Preserve legacy wire indices and route concurrent V4 responses and background requests by correlation and stable tab ID.

crates/ocs_plugin_api/src/ipc/v4/server.rs

@@ -27,7 +27,11 @@ macro_rules! vlog {
 /// A frame delivered from the V4 reader thread to the host main thread.
 pub enum HostIncoming {
     Response { id: u64, payload: crate::ipc::protocol::HostResponse },
-    Request { id: u64, payload: Box<crate::ipc::protocol::PluginRequest> },
+    Request {
+        id: u64,
+        tab_id: Option<u64>,
+        payload: Box<crate::ipc::protocol::PluginRequest>,
+    },
 }
 
 /// Token-bucket rate limiter.
@@ -117,9 +121,17 @@ pub fn run_host_reader_thread(
                         }
                     }
                 }
-                Ok(PluginToHostV4::Request { id, payload }) => {
+                Ok(PluginToHostV4::Request {
+                    id,
+                    tab_id,
+                    payload,
+                }) => {
                     if incoming
-                        .send(HostIncoming::Request { id, payload: Box::new(payload) })
+                        .send(HostIncoming::Request {
+                            id,
+                            tab_id,
+                            payload: Box::new(payload),
+                        })
                         .is_err()
                     {
                         break;