Transmission #029: Implementation plan for stats, achievements, quest boards

Added implementation/plan.md under .agents/planning/2026-04-22-stats-achievements-quests/implementation/: twelve incremental, demoable steps from power-of-ten math through inventory tabs, quest boards, full completion loop, remaining stat hooks, and a polish pass — each with tests wired in (no standalone “testing only” step). Checklist at top of the plan tracks delivery.