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/pointers.rs

@@ -2,6 +2,8 @@
 //! `scripts/check-runbook.sh` checks that the entries exist; these tests check that the messages
 //! name them. Do not edit.
 
+#[path = "support/serving.rs"]
+mod serving;
 mod support;
 
 use std::path::Path;
@@ -162,20 +164,10 @@ fn healthy_routes(server: &FakeServer) {
     );
 }
 
-/// Waits for `loopd serve` to bind its socket, stops it, and returns what it printed.
-fn stderr_once_serving(mut child: Child, socket: &Path) -> String {
-    let mut up = false;
-    for _ in 0..200 {
-        if socket.exists() {
-            up = true;
-            break;
-        }
-        std::thread::sleep(Duration::from_millis(50));
-    }
-    child.kill().unwrap();
-    let output = child.wait_with_output().unwrap();
-    let stderr = String::from_utf8_lossy(&output.stderr).to_string();
-    assert!(up, "the socket never appeared; stderr: {stderr}");
+/// Waits for `loopd serve` to print "serving on", stops it, and returns what it printed.
+fn stderr_once_serving(child: Child) -> String {
+    let (up, stderr) = serving::stderr_once_serving(child);
+    assert!(up, "loopd never said it was serving; stderr: {stderr}");
     stderr
 }
 
@@ -218,8 +210,7 @@ fn serve_without_a_broker_says_so_once_with_the_entry() {
     let server = FakeServer::start();
     healthy_routes(&server);
     let config = config_file(&home, &server, 2, "");
-    let socket = home.dir.join("run/loop/loop.sock");
-    let stderr = stderr_once_serving(serve(&config), &socket);
+    let stderr = stderr_once_serving(serve(&config));
     let lines: Vec<&str> = stderr
         .lines()
         .filter(|l| l.contains("no tool broker is configured"))
@@ -240,8 +231,7 @@ fn serve_with_a_broker_socket_does_not_say_it() {
         home.dir.join("run/loop-broker/broker.sock").display()
     );
     let config = config_file(&home, &server, 2, &extra);
-    let socket = home.dir.join("run/loop/loop.sock");
-    let stderr = stderr_once_serving(serve(&config), &socket);
+    let stderr = stderr_once_serving(serve(&config));
     assert!(!stderr.contains("no tool broker"), "{stderr}");
     assert!(stderr.contains("serving on"), "{stderr}");
 }