feat(hud): cache savings badge (Wave 2-C)

[JeremyDev87]

Quantifies the 90% discount on cache_read_input_tokens so the HUD can surface "💰$4.56 saved". Part of #1464.

[JeremyDev87]

Implemented and merged via #1485 (Wave 0-3 integrator) and #1487 (Wave 3b follow-up). Module lives in packages/claude-code-plugin/hooks/lib/. Closing as complete.