small: the page-load budget lives in `src/perf/page_budget.ts` as plain data #11360

Remainder of #11320. The budget table is in test/helpers/page_budget.ts; the box asks for it in one small plain-data module under src/perf/ so a page is added in one line and the production probe can read it too.

Done:

• [x] src/perf/page_budget.ts exports the table (page path, statement ceiling, median ms) as plain data; the test helper imports it.
• [x] test/page-budget.test.ts still passes; no budget value changes.

Files: those two plus the test import.

The bare move trips test/unreached.test.ts: a module under src/ that only tests import is unreached. Pull #11366 lists src/perf/page_budget.ts in scripts/unreached-allow.json for now; the entry leaves the list on its own (the test fails while it stays listed) once a production probe imports the table. A reviewer asked that a person decide whether that allow-list entry is acceptable, rather than the pull inventing a consumer: please weigh it when merging. A probe that reads the table needs a signed-in session, since the budgets are counted signed in (/map redirects signed out).

Reviewed pull #11366 at its head: ticked src/perf/page_budget.ts exports the table (page path, statement ceiling, median ms) as plain data; the test helper imports it.; test/page-budget.test.ts still passes; no budget value changes..