review: loopd prints every startup notice before it binds; tests wait for "serving on"

The serve tests waited for the socket file, then killed `loopd` and read its stderr, but the
socket is bound before its mode is set and before "serving on" and the broker notice are printed,
so a kill in between lost those lines (`pointers` failed once under load in M5c-1 task 15). The
broker notice now comes before the bind, so "serving on" is the last startup line, and the tests
wait for it (`tests/support/serving.rs`).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/loopd/tests/serve.rs

@@ -1,10 +1,11 @@
 //! Tests for the `loopd serve` command. Do not edit.
 
+#[path = "support/serving.rs"]
+mod serving;
 mod support;
 
 use std::os::unix::fs::PermissionsExt;
 use std::process::{Command, Stdio};
-use std::time::Duration;
 use support::{FakeServer, Home, Reply, tokenizer};
 
 fn config_file(home: &Home, server: &FakeServer, expect_slots: u32) -> std::path::PathBuf {
@@ -56,25 +57,16 @@ fn serve_runs_the_self_test_then_binds_the_socket_with_mode_0600() {
     healthy_routes(&server);
     let config = config_file(&home, &server, 2);
     let socket = home.dir.join("run").join("loop").join("loop.sock");
-    let mut child = Command::new(env!("CARGO_BIN_EXE_loopd"))
+    let child = Command::new(env!("CARGO_BIN_EXE_loopd"))
         .args(["serve", "--config"])
         .arg(&config)
         .stderr(Stdio::piped())
         .spawn()
         .unwrap();
-    let mut up = false;
-    for _ in 0..200 {
-        if socket.exists() {
-            up = true;
-            break;
-        }
-        std::thread::sleep(Duration::from_millis(50));
-    }
+    // "serving on" comes after the mode is set; the socket stays after loopd is killed.
+    let (up, stderr) = serving::stderr_once_serving(child);
+    assert!(up, "loopd never said it was serving; stderr: {stderr}");
     let mode = std::fs::metadata(&socket).map(|m| m.permissions().mode() & 0o777);
-    let _ = child.kill();
-    let output = child.wait_with_output().unwrap();
-    let stderr = String::from_utf8_lossy(&output.stderr);
-    assert!(up, "the socket never appeared; stderr: {stderr}");
     assert_eq!(mode.unwrap(), 0o600, "{stderr}");
     assert!(stderr.contains("selftest: ok"), "{stderr}");
     assert!(stderr.contains("serving on"), "{stderr}");