fix(router): allow tier upgrades despite cache cost

[lcp2021211]

Summary

• bypass cache-aware cost comparison when the judge requests a tier upgrade
• preserve the existing cache-cost comparison for strict tier downgrades
• add regression coverage for upgrades and both downgrade outcomes

Closes #594

Testing

• TypeScript build passes on Node.js 22.13.0
• new cache-aware switching tests: 3 passed
• router test suite: 20 passed

The full local suite otherwise reported two unrelated EPERM failures because the sandbox cannot listen on 127.0.0.1; three following tests were cancelled.