test: add seeded-gap integration tests for mutation testing

Add three integration tests that validate the mutation testing system
catches real test suite gaps:

- Access control gap: weak tests only check owner path, never test
  non-owner rejection. Verifies require(true) mutation survives.
- Boundary condition gap: withdrawal tests never test exact balance.
  Verifies >= to > mutation survives.
- Arithmetic gap: test uses input where * and + produce same result
  (2*2 == 2+2). Verifies operator swap mutation survives.

Amp-Thread-ID: https://ampcode.com/threads/T-019c47bc-5807-7721-9051-7028de802b89
Co-authored-by: Amp <amp@ampcode.com>