Spawns stand on a surface

Every overworld cell is one of three surfaces (Chunk.getCellSurface, SpawnSurface in src/data/ids/biome.ts):

• Land is any cell that is not water. An open sea's islands are land.
• Water is a water cell in any biome, the open sea included.
• Ice is a water cell in a biome whose water is drawn frozen (isIceBiome). Ice is walked like ground.

A pool per surface, cut by what stands there

A biome's file in src/data/biome/pools/ holds a land pool and may hold water and ice pools. A cell draws from its own surface's pool, cut to what can stand on that surface (getSpawnPool), so a water pool lists everything met on water and its weights are shares of water rolls. Nothing is borrowed from another surface.

A surface with no pool of its own draws from the land pool, cut the same way: a lake in a field meets the field's swimmers and fliers. Ice with no pool of its own is walked like land outright (drawnSurface in chunk-snapshot.ts), and a volcano's water is lava that nothing stands on.

getBiomeRoster still merges a biome's surfaces, a species listed twice counting once at its heavier weight, for what reads a biome as a whole: the dex's habitats, the egg pool, the landmarks' ranks.

Three kinds

getSpawnClass sorts every species, in this order:

• Flying: Flying type, the Flying egg group, or Levitate in its ability pool. Stands on ground and water alike.
• Water: Water type, or the Water 1, 2 or 3 egg group. Stands only on water, unless its habitat is Amphibious, which lets it onto ground and ice as well.
• Ground: everything else. Stands only on ground and ice.

Two hand-picked lists in src/data/biome/__create.ts override the rules: WATER_ONLY_FLIERS (Gyarados, Mantine, Mantyke) are water rather than flying, and DRY_LAND_SPECIES (Skorupi, Drapion) are ground despite their egg group. Add to them rather than bending the rules.

fitsSurface states the placement rule. Caves cut their own pool the same way.

Phenomena draw from a kind

Rippling water startles the water kind, a flying shadow the flying kind and a dust cloud the ground kind (getClassPool), whatever surface the landmark stands on. A grotto takes whatever stands on the ground.

Amphibious

habitat: Amphibious (in the species' world/ YAML) only matters for the water kind. Mark a water species amphibious when it is met on land as readily as in the water: frogs, turtles, crabs, otters, seals, the water starters. Fish, jellies, shellfish and whales stay water-only, and so does a water species with no habitat at all.