Performance trace, load test and chapter read-ahead

A switchable trace (DATA/perf/trace.on or PERF_TRACE=1) records every request's
phases (stat, open, first read, disk vs network wait), browser resource timings,
page-turn waits and flow stalls, a cold storage probe, event-loop lag, scans and
scraper calls as JSONL. tools/perf-report.js says which phase slow loads spend
their time in; tools/perf-bench.js simulates readers.

READ_AHEAD (default on) reads an opened chapter's pages, then the next chapter's,
into the OS cache so a NAS's cold reads stop reaching the browser.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/reader.js

@@ -74,6 +74,9 @@
     D = payload;
   }
   catch (e) { canvas.textContent = ''; canvas.append(h('p', { class: 'loading' }, e.message + ' · ', h('a', { href: './' }, 'Library'))); return; }
+  // Timings for the server's performance trace, only while an admin has it switched on.
+  const perf = D.perf && window.PerfTrace ? PerfTrace.start('read') : null;
+  let perfFlow = null;
 
   // An already-open chapter also closes when a window ends or an unlock expires.
   const checkReadingAccess = async () => {
@@ -250,11 +253,13 @@
     if (mode === 'flow') {
       els = list.map(p => canvas.appendChild(pageEl(p, true)));
       endEl = canvas.appendChild(endCard());
+      if (perf) { if (perfFlow) perfFlow.disconnect(); perfFlow = perf.watchFlow(stage, els); }
     } else if (atEnd) {
       endEl = canvas.appendChild(endCard());
     } else {
       shown = mode === 'page' ? [cur] : spreads[spreadIdx];
       els = shown.map(k => canvas.appendChild(pageEl(list[k])));
+      if (perf) perf.view(els.map(el => el.firstChild), { mode, page: list[shown[0]].i });
       // The pages on screen get the bandwidth first, then the next ones are fetched.
       const token = ++buildToken, waiting = els.map(el => el.firstChild).filter(img => !img.complete);
       Promise.all(waiting.map(img => new Promise(done => {