Cloud Lighting / Dynamic Self-Shadow Update

- Added CLOUD_SELF_SHADOW, CLOUD_TOP_LIGHT and CLOUD_UNDERSIDE_DARKNESS controls.
- Cloud self-shadow direction follows the current sun vector instead of staying fixed.
- Shadow sampling distance increases as the sun gets lower, creating a longer directional shadow during morning/evening.
- When viewed from below the cloud layer, direct sunlight is reduced on the visible underside.
- Sun-facing cloud regions receive stronger direct light and softer silver lining.
- Added a subtle cool moonlight fill at night.
- Existing cloud wind animation and horizon curvature remain unchanged.
