feat(hud): cost velocity indicator (Wave 2-B)

[JeremyDev87]

Session spend rate (USD/min) with trend glyphs: 🔥 hot, ↗ rising, → steady, 💤 idle. Stateless — uses stdin cost + duration directly. 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.