/system shows each first-opened page's measured median beside its budget, read from PAGE_BUDGETS #11457

Intent: speed is first class: the owner sees which page is over its budget today, without running a script.

Done:

• [ ] PAGE_BUDGETS (src/perf/page_budget.ts) is read by a /system section: one row per page, its budget (ms, SQL statements) and its measured median from src/perf/record.ts; a page over budget is one plain line, not a red banner
• [ ] the same rows answer GET /api/v1 system data the page reads and the MCP tool that already reports system, same words (no new tool; use an action or kind on an existing one, docs/MCP.md)
• [ ] test/mcp-size.test.ts still passes; a test fails if the table and the section drift
• [ ] module leaves scripts/unreached-allow.json

Files: src/perf/page_budget.ts, src/web/pages/system.ts (one mount), a new src/web/pages/system-budget.ts, scripts/unreached-allow.json, docs/features/page-budget.md, tests. Risks: reading the perf record must stay a cached read; /system must not slow down. Suggested brief: scout.