feat(router): add explicit kilo gateway lanes

[typelicious]

Summary

• add explicit Kilo Sonnet and Opus lanes plus Kilo-specific routing fit scoring
• clarify BLACKBOX as a low-cost burst path instead of assuming it is always free
• update roadmap and changelog so this block becomes the release target anchor

Testing

• rtk ruff check faigate/lane_registry.py faigate/provider_catalog.py faigate/registry.py faigate/router.py tests/test_api_hardening.py tests/test_provider_catalog.py tests/test_routing_dimensions.py
• env PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_provider_catalog.py tests/test_routing_dimensions.py tests/test_api_hardening.py -k 'kilo or blackbox or route_preview or route_summary'
• rtk git diff --check --cached