feat: implement endpoint link and exit handling

test/remote_child_endpoint.zig

@@ -259,7 +259,13 @@ const StdioEndpoint = struct {
                     return tp.exit_error(error.UnknownWireId, null);
             },
             .transport_error => |te| return tp.exit(te.reason),
-            .link, .exit => return tp.exit_error(error.UnexpectedMessage, null),
+            .exit => |e| {
+                // Remote side notifying that a linked actor has exited.
+                // Forward to the local actor with that wire ID (best-effort).
+                if (self.wire_ids.get(e.id)) |actor|
+                    actor.send(.{ "exit", e.reason }) catch {};
+            },
+            .link => return tp.exit_error(error.LinkNotSupported, null),
         }
     }