dev:all stops both sides when the server's port is taken

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jm2auWk6RP71CjaAb4FMoG

scripts/dev-all.mjs

@@ -1,5 +1,6 @@
 // The game server and the site together, for playing locally: one terminal,
-// each line marked with its side, and Ctrl-C (or either one stopping) stops both.
+// each line marked with its side, and Ctrl-C (or either one stopping or
+// failing to get its port) stops both.
 import { spawn } from 'node:child_process';
 
 const sides = [
@@ -13,7 +14,14 @@ const children = sides.map(([name, args]) => {
 		stream.on('data', (chunk) => {
 			const lines = (rest + chunk).split('\n');
 			rest = lines.pop();
-			for (const line of lines) console.log(`[${name}] ${line}`);
+			for (const line of lines) {
+				console.log(`[${name}] ${line}`);
+				// A watcher outlives the crash it watches, so a port already taken would leave one side running alone.
+				if (line.includes('EADDRINUSE')) {
+					console.log(`[${name}] its port is taken: is another copy already running?`);
+					stop(1);
+				}
+			}
 		});
 	}
 	child.on('exit', (code) => stop(code ?? 0));