Transmission #028: Detailed design for stats, achievements, and quest boards POC

Authored detailed-design.md for the planning slice under .agents/planning/2026-04-22-stats-achievements-quests/design/: standalone spec covering the seven stats, power-of-ten achievements, HelerionInventoryBridge grant path, client random loot boxes, quest board determinism, inventory tab extensions, toasts, and provider interfaces for a future API swap.