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/main.rs

@@ -6,6 +6,24 @@ mod surface;
 
 use anyhow::Result;
 
+/// Test-only support shared across the crate's test modules.
+#[cfg(test)]
+pub(crate) mod test_support {
+    use std::sync::{Mutex, MutexGuard};
+
+    /// Process-wide serialization lock for the heavy socket/PTY integration tests.
+    /// These bind sockets and spawn real PTYs/processes; running several at once on a
+    /// many-core box starves each other's tasks and trips timing assumptions. Unit
+    /// tests stay parallel; only guarded integration tests serialize on this lock.
+    static SERIAL: Mutex<()> = Mutex::new(());
+
+    /// Acquire the serial lock for the duration of a test. Poison-tolerant so one
+    /// panicking test does not cascade-fail the rest.
+    pub(crate) fn serial() -> MutexGuard<'static, ()> {
+        SERIAL.lock().unwrap_or_else(|e| e.into_inner())
+    }
+}
+
 #[tokio::main]
 async fn main() -> Result<()> {
     let arg = std::env::args().nth(1);