Fogged

  Core mechanic
  - Breathing boundary at a configurable Y — below it you breathe as if underwater and lose air/drown
  - Boundary height moves over time: day→height schedule (interpolated), time-of-day offset, optional back-and-forth cycle
  - As underwater fire can't be in thick fog (lava→obsidian, torch breaks)
  - Murk fog below the boundary: short dense scene fog + screen fog tinted to the plane colour (looks like low render distance / underwater)

  Separation plane (the visible "surface")
  - Large camera-centred coloured plane at the boundary, writes depth so deep far-side water/geometry is hidden
  - World-anchored foam rings around blocks/entities/flowing water crossing the line, plus animated foam particles
  - Cold-vapour mist: stacked terraced sheets over the plane
  - Soft depth-fade edges against shores/blocks
  - Near-player + per-entity dissolve holes so crossing mobs/players poke through instead of being sliced
  - Sky/sun/moon hidden under the plane; hides clouds while submerged

  Blocks / items
  - Fog detector — emits redstone scaled by how much of its column is under
  the fog. Stacks into a tower (base + extensions, up to 15) by right-clicking with a detector item

  Mobs
  - Fog lurker — burrowing rat that stalks under-fog players, try to not annoy it
  - Murk suffocation — non-allowed mobs can't spawn and take damage under the fog (configurable allow-list)

  Flora (in development, off by default)
  - Fog moss — rots/spreads under the plane, cold/temperate/warm biome variants
  - Foggy grass — grows on moss (cross-shaped placeholder models)