Read-ahead per reader; compose passes READ_AHEAD and PERF_TRACE; report fixes

Each user gets an own read-ahead queue, served in turn, so two people reading
at once no longer cancel each other's chapter. The report rounds phase times
and measures the quiet time before a slow storage probe correctly.

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

.env.example

@@ -20,6 +20,10 @@ READING_TIMEZONE=UTC
 READING_QUIZ_UNLOCK_MINUTES=60
 # Optional deployment-managed JSON file under DATA_PATH, e.g. /data/reading-quiz.json.
 # READING_QUIZ_CATALOG_PATH=/data/reading-quiz.json
+# Read an opened chapter into the OS cache ahead of the browser (1 = on, the default); see docs/performance.md.
+# READ_AHEAD=1
+# Performance trace from the start (also switchable at runtime: touch DATA_PATH/perf/trace.on).
+# PERF_TRACE=0
 PROXY_NETWORK=mangacircus-edge
 # Extra Compose files, ':'-separated (docs/deployment.md), e.g. HTTPS with the bundled Caddy:
 # COMPOSE_FILE=docker-compose.yml:deploy/compose.https.yml