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>

Dockerfile

@@ -11,6 +11,7 @@ COPY tools ./tools
 COPY scraper ./scraper
 COPY peering ./peering
 COPY runtime ./runtime
+COPY perf ./perf
 RUN mkdir -p /library /data && chown node:node /data /library
 ENV PORT=8080 LIBRARY=/library DATA=/data TZ=UTC
 EXPOSE 8080