The currently active scaler profile, or null if no profile has been loaded.
Only one scaler profile is active at a time. Call IAdaptivePerformanceSettings.LoadScalerProfile to
activate one. The getter does not auto-bind to the default profile: a previous lazy
initialization here ran before the scalers were actually bound to the profile, which
caused LoadScalerProfile's "already loaded" early-out (a Name comparison against
ActiveScalerProfile) to short-circuit on first call and skip ApplyScalerProfileToAllScalers,
leaving every default scaler with its inline-default settings (base names, default bounds,
disabled) until something else triggered a re-apply. Returning null until LoadScalerProfile
runs makes "no profile loaded" an honest, observable state and lets the early-out work.