Pebble

Water and aquatic-plant rendering fixes from #12. Thanks to @MEmin00 for the detailed reports.

Fixed

• Sharp water color seam — water/grass/foliage tint is now blended over a 7×7 neighborhood instead of a hard per-block biome lookup, so the color grades smoothly across biome borders instead of showing a line.
• Seagrass/kelp air pockets — waterlogged plants (seagrass, kelp, coral, sea pickles) now render the water volume around them, so they're properly submerged instead of sitting in un-rendered gaps.
• Notch in the water surface around seagrass/kelp poking the surface — the surrounding water no longer bulges to a full block, so the surface stays flat.
• Breaking an aquatic plant now leaves water, not an air pocket (player breaks, natural pops, and piston/explosion breaks).
• Breaking one seagrass no longer destroys its neighbors — the water that filled the broken cell was spreading and washing away adjacent replaceable plants; spreading water now leaves waterlogged plants intact.

All 456 golden tests pass; the mesh and worldsim goldens were re-baselined for the render/fluid changes.