Sky Whales

Sky whales now spawn over oceans.

• Removed the old high-altitude spawn band (Y 200-310) that stopped natural ocean spawns on lifted-terrain worlds. The spawn rule is now tied to the ocean surface and self-sufficient (no external datapack required).
• Naturally-spawned whales are lifted into open air, so they appear airborne over the sea rather than half-submerged.
• Whales linger longer before despawning (despawn distance 64 -> 120).