fix: resolve scene dependency errors and missing UID in main.tscn

- WaterBody.tscn, WaterfallBody.tscn: gdshader files were referenced
  with type="Material" instead of type="Shader". Wrapped each
  shader in a ShaderMaterial sub-resource so Godot can load them.
- ChimneySmoke.tscn: reordered sub-resources so StandardMaterial3D
  is defined before QuadMesh that references it.
- WaterfallMist.tscn: fixed node type GpuParticles3D -> GPUParticles3D
  (case-sensitive class name in Godot 4.7).
- jajce_world.gd: added explicit Vector3 type annotation to fix
  'Cannot infer the type of pos variable' parse error.
- main.tscn.uid: created missing UID mapping file. The scene header
  declared uid="uid://rs3hv73svbpa" but no .uid file existed,
  causing Godot to reject the scene as unsupported format.

world/jajce/WaterfallMist.tscn

@@ -16,7 +16,7 @@ alpha_curve = SubResource("Curve_alpha")
 emission_shape = 1
 emission_box_extents = Vector3(2.5, 1.0, 0.5)
 
-[node name="WaterfallMist" type="GpuParticles3D"]
+[node name="WaterfallMist" type="GPUParticles3D"]
 emitting = true
 amount = 48
 lifetime = 2.5