test(daemon): serialize heavy socket/PTY integration tests

Process-wide serial lock around the socket-binding and PTY-spawning
integration tests in spaceshd. Running several at once on a many-core
box starved each other's async tasks and tripped timing assumptions,
causing ~1/10 flakes under cargo test --workspace. Unit tests stay
parallel. 0/20 spaceshd + 0/5 workspace runs after the change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spaceshd/src/server.rs

@@ -323,6 +323,7 @@ mod tests {
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn open_new_surface_attach_streams_output() {
+        let _serial = crate::test_support::serial();
         let dir = tempdir_path();
         let sock = dir.join("sock");
         let sock_for_task = sock.clone();
@@ -360,6 +361,7 @@ mod tests {
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn unknown_surface_returns_not_found() {
+        let _serial = crate::test_support::serial();
         let dir = tempdir_path();
         let sock = dir.join("sock");
         let sock_for_task = sock.clone();
@@ -401,6 +403,7 @@ mod tests {
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn reattach_returns_snapshot_with_prior_output() {
+        let _serial = crate::test_support::serial();
         let dir = tempdir_path();
         let sock = dir.join("sock");
         let sock_for_task = sock.clone();