test(xy): logs --tail and --follow

Fix a deadlock in the log-stream handler that caused all logs
requests to hang: Connection used a single Mutex<JsonFramed> for
both reads and writes, so the serve loop holding the read lock
blocked the spawned notification task from writing.  Split
Connection into separate reader and writer mutexes.

Also fix a response/notification ordering race: the log task now
waits for an explicit ready signal sent by serve after writing the
LOGS response, ensuring notifications never arrive at the client
before their initiating response.

crates/xy-ipc/src/client.rs

@@ -1,7 +1,7 @@
 use crate::envelope::{Incoming, Notification, Request};
 use crate::framing::JsonFramed;
-use serde::de::DeserializeOwned;
 use serde::Serialize;
+use serde::de::DeserializeOwned;
 use std::path::Path;
 use thiserror::Error;
 use tokio::net::UnixStream;