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

scripts/core/item_db.gd

@@ -14,11 +14,14 @@ const ICON_SIZE := 96
 var items: Dictionary = {}     # id -> definition Dictionary
 var recipes: Dictionary = {}   # id -> recipe Dictionary (insertion order kept)
 var icons: Dictionary = {}     # id -> Texture2D
+## Fishing by water type (data/fish.json): {"modifiers": {...}, "waters": {type: {...}}}.
+var fishing: Dictionary = {"modifiers": {}, "waters": {}}
 var _item_mat: StandardMaterial3D
 
 func _ready() -> void:
 	_load("res://data/items.json", "items", define_item)
 	_load("res://data/recipes.json", "recipes", define_recipe)
+	_load_fishing("res://data/fish.json")
 	if DisplayServer.get_name() != "headless":
 		_render_icons.call_deferred()
 
@@ -51,6 +54,31 @@ func define_recipe(r: Dictionary) -> void:
 	r.station = String(r.get("station", ""))
 	recipes[r.id] = r
 
+func _load_fishing(path: String) -> void:
+	var data = JSON.parse_string(FileAccess.get_file_as_string(path))
+	if typeof(data) != TYPE_DICTIONARY or not data.has("waters"):
+		push_error("ItemDB: could not parse %s" % path)
+		return
+	fishing.modifiers = data.get("modifiers", {})
+	for water in data.waters:
+		define_water(water, data.waters[water])
+
+## Registers (or replaces) the fish for one water type. Same shape an addon
+## would use to add, say, a "swamp" water with its own catch.
+func define_water(water: String, d: Dictionary) -> void:
+	var fish: Array = []
+	for f in d.get("fish", []):
+		if not items.has(f.get("id", "")):
+			push_error("ItemDB: fish '%s' in water '%s' is not an item" % [f.get("id", "?"), water])
+			continue
+		fish.append({"id": f.id, "weight": float(f.get("weight", 1)), "dawn_dusk": float(f.get("dawn_dusk", f.get("weight", 1)))})
+	var w: Array = d.get("bite_wait", [5.0, 14.0])
+	fishing.waters[water] = {"min_depth": float(d.get("min_depth", 0.5)), "bite_wait": [float(w[0]), float(w[1])], "fish": fish}
+
+## The fishing table for a water type ({} = nothing to catch there).
+func water_fishing(water: String) -> Dictionary:
+	return fishing.waters.get(water, {})
+
 func get_item(id: String) -> Dictionary:
 	return items.get(id, {})