PERF.13 Website: /tutorials spends 290 ms of PHP parsing every help Markdown file on every request

Measured 2026-11-08 (bin/admin-perf.php /tutorials): 292 ms, only 7 queries (2 ms); the time is Help::forApp x2 and Help::common x2 (core/classes/Help.php: glob the help folder, read and parse every .md topic, the shots map) for the index page. /tutorials/{app} and the in-app help panel pay the same per request.

Plan: compile each build's help once (bin/build-help.php already exists for the compiled help.js) and cache Help::forApp/common per build stamp as JSON in storage/cache (or ship the parsed index with the build), so a request reads one file. Target under 30 ms.