The Abyss v12 client visuals:
- Vanilla underwater screen overlay remains disabled in abyss:the_abyss.
- Fog depth curve is gentler than v11: ~150 blocks near surface, ~38 blocks at abyss floor.
- Looking upward increases fog range up to ~30% and shifts fog brighter/bluer to preserve vertical orientation.
- Deep fog remains dark but no longer crushes visibility as quickly.
- Selective "glowing objects ignore fog" is not implemented because vanilla fog is applied uniformly after shading; emissive content still benefits from the longer far plane.

Worldgen:
- Adds sparse, extremely tall kelp columns from the abyssal floor toward the Y=400 surface.
- Uses Hybrid Aquatic bull kelp and sargassum plus vanilla kelp.
- Columns are truncated by water availability, keeping a proper head at the top.
