Add distributed limiter readiness proof

[WaffleBits]

Summary

• add a public-safe distributed limiter readiness artifact for request-count and estimated-input-token budgets
• map configured model policies to Redis fixed-window and Envoy global-rate-limit descriptor shapes
• document the migration boundary honestly: readiness evidence only, not a live Redis or Envoy deployment

Validation

• python -m unittest discover -s tests
• python -m gateway.distributed_limiter --output artifacts\distributed-limiter-evidence.json
• JSON parse for checked gateway artifacts
• git diff --check
• artifact leakage scan for payload, identity, secret, and prompt fields