onMount(() => {
        /* the boot census starts here. Every command the app issues between
           this line and the first frame is counted, and the mock walk holds
           a budget against the count — the point is to catch a round that
           loads the world at boot, not to police milliseconds. */
        boot.t0 = performance.now();
        try { performance.mark("ss:boot"); } catch { /* a webview without the entry types */ }
        loadPrefs();
        bootNav();