fix(test): eliminate race in exit-on-ctrlc output ordering

Move "ctrl-c received" print from the signal handler to the main thread
(after OnceLock::wait), so it always executes after the milestone flush
println!(). Previously, std::process::exit(0) in the handler could
terminate before output reached the PTY, causing flaky snapshot diffs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

crates/vite_task_bin/src/vtt/exit_on_ctrlc.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 /// exit-on-ctrlc
 ///
 /// Sets up a Ctrl+C handler, emits a "ready" milestone, then waits.
@@ -25,19 +27,25 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
         }
     }
 
-    ctrlc::set_handler(move || {
-        use std::io::Write;
-        let _ = write!(std::io::stdout(), "ctrl-c received");
-        let _ = std::io::stdout().flush();
-        std::process::exit(0);
+    let ctrlc_once_lock = Arc::new(std::sync::OnceLock::<()>::new());
+
+    ctrlc::set_handler({
+        let ctrlc_once_lock = Arc::clone(&ctrlc_once_lock);
+        move || {
+            let _ = ctrlc_once_lock.set(());
+        }
     })?;
 
+    // Print "ready" before the milestone so it is always visible in the screen
+    // capture at milestone time. The milestone's mark_milestone() flushes stdout
+    // first, ensuring "ready" reaches the PTY before the OSC 8 sequence. The
+    // trailing println!() provides a newline to flush milestone bytes through
+    // line-buffered writers (labeled log mode).
+    print!("ready");
     pty_terminal_test_client::mark_milestone("ready");
-    // Print a newline so the milestone bytes get flushed through line-buffered
-    // writers (labeled/grouped log modes).
     println!();
 
-    loop {
-        std::thread::park();
-    }
+    ctrlc_once_lock.wait();
+    println!("ctrl-c received");
+    Ok(())
 }

crates/vite_task_bin/tests/e2e_snapshots/fixtures/ctrl-c/snapshots/ctrl-c terminates running tasks (cached).snap

@@ -5,7 +5,8 @@ expression: e2e_outputs
 > vt run dev-cached
 @ expect-milestone: ready
 $ vtt exit-on-ctrlc
+ready
 @ write-key: ctrl-c
 $ vtt exit-on-ctrlc
-
+ready
 ctrl-c received

crates/vite_task_bin/tests/e2e_snapshots/fixtures/ctrl-c/snapshots/ctrl-c terminates running tasks (labeled).snap

@@ -5,9 +5,8 @@ expression: e2e_outputs
 > vt run --log=labeled dev
 @ expect-milestone: ready
 [ctrl-c-test#dev] $ vtt exit-on-ctrlc ⊘ cache disabled
-[ctrl-c-test#dev] 
+[ctrl-c-test#dev] ready
 @ write-key: ctrl-c
 [ctrl-c-test#dev] $ vtt exit-on-ctrlc ⊘ cache disabled
-[ctrl-c-test#dev] 
-
+[ctrl-c-test#dev] ready
 [ctrl-c-test#dev] ctrl-c received

crates/vite_task_bin/tests/e2e_snapshots/fixtures/ctrl-c/snapshots/ctrl-c terminates running tasks.snap

@@ -5,7 +5,8 @@ expression: e2e_outputs
 > vt run dev
 @ expect-milestone: ready
 $ vtt exit-on-ctrlc ⊘ cache disabled
+ready
 @ write-key: ctrl-c
 $ vtt exit-on-ctrlc ⊘ cache disabled
-
+ready
 ctrl-c received