Transmission #047: PDD research notes for skill and character leveling

Documented a research pass for the leveling POC: how PlayerStatTracker, StatMilestoneEvaluator, and ProgressionOrchestrator drive achievements and loot boxes; how InventoryPanelUI builds tabs; and concrete extension points for skill levels, character-level stat-point claims, and new Skills/Base stats tabs. Artifacts live under .agents/planning/2026-04-23-skill-character-levelling/research/.