Transmission #032: PlayerStatTracker (implementation Step 2)

Added Helerion.Progression.PlayerStatTracker (session counters, Add, Get, Reset, StatValueChanged), PlayerStatTrackerProvider (singleton MonoBehaviour with DontDestroyOnLoad, auto-created after scene load only if no provider exists yet), and PlayerStatTrackerDebugKeys (F9 adds a punch + logs). Edit Mode tests cover accumulation, fractional distance, events, reset, and invalid Add inputs. Implementation plan Step 2 is checked off.