fix: serialize milestone tests on musl to prevent SIGSEGV

The PTY_LOCK in pty_terminal serializes spawn and FD cleanup, but
interleaved reads/writes between two live Terminals can still trigger
SIGSEGV in musl internals. Add a test-level mutex so milestone tests
(which maintain long-lived interactive PTY sessions) don't overlap.

Author: claude

crates/pty_terminal_test/tests/milestone.rs

@@ -1,14 +1,26 @@
 use std::io::Write;
+#[cfg(target_env = "musl")]
+use std::sync::Mutex;
 
 use ntest::timeout;
 use portable_pty::CommandBuilder;
 use pty_terminal::geo::ScreenSize;
 use pty_terminal_test::TestTerminal;
 use subprocess_test::command_for_fn;
 
+// On musl, concurrent PTY spawns + FD operations in the same process cause
+// SIGSEGV in musl internals. The crate-level PTY_LOCK serializes spawn and
+// drop, but interleaved reads/writes between two live Terminals can still
+// trigger the race. Serialize entire test bodies as a workaround.
+#[cfg(target_env = "musl")]
+static TEST_MUTEX: Mutex<()> = Mutex::new(());
+
 #[test]
 #[timeout(5000)]
 fn milestone_raw_mode_keystrokes() {
+    #[cfg(target_env = "musl")]
+    let _guard = TEST_MUTEX.lock().unwrap_or_else(|e| e.into_inner());
+
     let cmd = CommandBuilder::from(command_for_fn!((), |(): ()| {
         use std::io::{Read, Write, stdout};
 
@@ -77,6 +89,9 @@ fn milestone_raw_mode_keystrokes() {
 #[test]
 #[timeout(5000)]
 fn milestone_does_not_pollute_screen() {
+    #[cfg(target_env = "musl")]
+    let _guard = TEST_MUTEX.lock().unwrap_or_else(|e| e.into_inner());
+
     let cmd = CommandBuilder::from(command_for_fn!((), |(): ()| {
         use std::io::{Read, Write, stdout};