- data/fish.json: fish, weights, dawn/dusk odds, bite timing and minimum
depth per water type; ItemDB.define_water() registers them
- world.water_at(x, z) answers lake / sea / dry land; the float uses it,
so a worldlet with a sea only needs a "sea" entry
- Clear messages for shallow water and water with nothing to catch
- Lake fish: more of them, bigger, nearer the shore, jumping with splash rings
- docs/WORLDLETS_DESIGN.md: proposal for data-defined world types
