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>

AGENTS.md

@@ -12,6 +12,7 @@ Guide for contributors and coding agents. Users who only want to run it: [docs/d
 | `scraper/` | Download queue daemon (`service.js`, `daemon.js`), source adapters and parsers, the publisher that writes the library, metadata lookups |
 | `peering/` | Optional server-to-server sharing (gateway, client, adapter) |
 | `runtime/` | In-container supervisor and configuration |
+| `perf/` | Switchable performance trace and chapter read-ahead ([docs/performance.md](docs/performance.md)) |
 | `discovery/`, `greatest/` | Discovery Scout and the Greatest of All Time dashboard |
 | `tools/` | Demo library, health checks, maintenance tools (dry run by default), UI and Compose test drivers |
 | `deploy/` | `circus.sh` operations helper, HTTPS with Caddy |