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/read.html

@@ -16,6 +16,7 @@
 <script src="reading-clock.js" defer></script>
 <script src="keys.js" defer></script>
 <script src="moments.js" defer></script>
+<script src="perf.js" defer></script>
 <script src="reader.js" defer></script>
 </head>
 <body class="reader">