Edge-case fixture for pre-authorization cache isolation

[echowang1]

Concrete synthetic assertions beyond the reuse and growth cases already covered by PR 188:

• A cached authorization past its TTL is never reused, even when its amount still covers the estimate.
• Concurrent requests cannot consume the same single-use authorization twice.
• Cache entries stay isolated across model and request-path boundaries.
• A negative, non-numeric, or non-finite estimator result fails closed instead of enabling reuse.
• Underpayment is rejected; separately, an accepted overpayment records the actual settled amount without a second debit.

Gauntlet is a local-only V0.1 assertion harness, not production payment security.

Would these synthetic cache-isolation and settlement-edge assertions be useful alongside ClawRouter PR 188?