Fish per water type in data/fish.json; worldlets design

- 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

data/fish.json

@@ -0,0 +1,29 @@
+{
+  "_comment": "Fishing by water type. world.water_at() says which type a spot is; a type with no entry has no fish. Weights are relative. 'dawn_dusk' replaces the weight at dawn (05:00-08:30) and dusk (17:00-20:30). Bite waits are seconds, scaled by the modifiers.",
+  "modifiers": {
+    "dawn_dusk_wait": 0.6,
+    "rain_wait": 0.8,
+    "missed_wait": 1.3
+  },
+  "waters": {
+    "lake": {
+      "min_depth": 0.5,
+      "bite_wait": [
+        5.0,
+        14.0
+      ],
+      "fish": [
+        {
+          "id": "perch",
+          "weight": 78,
+          "dawn_dusk": 60
+        },
+        {
+          "id": "trout",
+          "weight": 22,
+          "dawn_dusk": 40
+        }
+      ]
+    }
+  }
+}