/system shows each first-opened page's warm median beside its budget, an over-budget page as one plain line #11578

Intent: the owner sees on /system which first-opened page is over its budget today, without running a script. Second half of #11544. Start fresh from main; touch only the files below.

Done:

• [ ] new src/web/pages/system-budget.ts renders a section from pageBudgetRows (src/perf/page_budget_read.ts): one row per page with its budget and measured median, every value escaped
• [ ] an over-budget page reads as one plain line in words ("map answers in 82 ms, budget 50 ms"), no red banner; a page with no samples reads "not opened in the last 15 minutes"
• [ ] src/web/pages/system.ts mounts it once, reading the in-memory perf record: no new SQL, /system's own statement count does not rise
• [ ] remove src/perf/page_budget_read.ts from scripts/unreached-allow.json
• [ ] a render test in test/page-budget-read.test.ts shows each page name and the over line
• [ ] one short paragraph in docs/features/page-budget.md; no other docs
• [ ] bun scripts/design-lint.ts, bun x tsc --noEmit and the touched tests pass in the sandbox before the push

Files: src/web/pages/system-budget.ts (new), src/web/pages/system.ts, test/page-budget-read.test.ts, scripts/unreached-allow.json, docs/features/page-budget.md. Risks: the earlier try's second park was the check's CPU limit (#11573), not this code; do not touch the MCP tool here. Suggested brief: scout.