feat: clean up local_actors/outbound tables when senders exit

The endpoint now links to each local actor when it is first assigned an
outbound wire ID, trapping the exit to remove stale entries from the
outbound and local_actors tables.

To preserve spawn_link semantics, the endpoint stores the spawner's
instance_id() at init time. Trapped exits from the spawner are always
re-propagated regardless of whether the spawner also appears as an outbound
sender.

test/remote_endpoint_test.zig

@@ -37,6 +37,7 @@ const TestActor = struct {
             endpoint.Args{
                 .allocator = args.allocator,
                 .argv = argv,
+                .spawner = thespian.self_pid().ref(),
             },
             endpoint.start,
             "endpoint",