test: assert default preset limit values match README

## Summary
Lock documented defaults for `presets.api`, `auth`, and `page` so README and code cannot drift.

## Tasks
In `test/presets/composite.test.ts` or new `test/presets/defaults.test.ts`:
- Assert `api` uses 100 / 15m (via behavior or exported config if available)
- Assert `auth` uses 5 / 15m
- Assert `page` uses 300 / 1m

If limits are not introspectable, test via controlled requests + memory storage counting.

## Skills needed
Vitest, reading `src/presets/index.ts`.

## Acceptance criteria
- [ ] Tests fail if defaults change without README update
- [ ] `npm test` green