chore: add web toolchain and infrastructure for Plan 3

public/index.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+// Front controller. The router and handler dispatch land in Task 13.
+// For now this returns a 200 with a stub so the dev server is reachable.
+http_response_code(200);
+header('Content-Type: text/plain; charset=utf-8');
+echo "BattleForge dev server up.\n";