Pilvia perf harness

A deployed app whose only job is to be measured. Driven by scripts/perf.sh in pilvia/app.

• /api/ping — four bytes, the isolate floor
• /ssr/page — a ~12.5 KB page rendered per request
• /static.html — the same bytes, served as an asset
• /ssr/heavy — the same renderer, 600 rows
• /api/json — 200 records through JSON.stringify
• /ssr/site/:slug — a parameterised route
• POST /api/echo — the request-body path

Every route renders from a seeded generator, so output is byte-stable across requests: a number that moves is the platform moving, not the data.