Fix terrain: add planar UVs, drop generate_tangents (no normal map needed)

scripts/world/terrain.gd

@@ -29,8 +29,10 @@ func _build() -> void:
 
 	var step := float(size) / float(resolution)
 	var half := size * 0.5
+	var uv_scale := 1.0 / float(size)
 
-	# Build a grid of quads, two triangles each.
+	# Build a grid of quads, two triangles each. Each vertex carries a planar UV
+	# (world-XZ mapped to 0..1) so the surface is ready for texturing later.
 	for z in range(resolution):
 		for x in range(resolution):
 			var x0 := x * step - half
@@ -43,11 +45,13 @@ func _build() -> void:
 			var p01 := Vector3(x0, _height(noise, x0, z1), z1)
 			var p11 := Vector3(x1, _height(noise, x1, z1), z1)
 
-			_add_tri(st, p00, p01, p11)
-			_add_tri(st, p00, p11, p10)
+			_add_tri(st, p00, p01, p11, half, uv_scale)
+			_add_tri(st, p00, p11, p10, half, uv_scale)
 
+	# Normals give the lighting its shape. Tangents are only needed for normal
+	# maps, which this low-poly material doesn't use — so we skip them (and avoid
+	# the "UVs required to generate tangents" error).
 	st.generate_normals()
-	st.generate_tangents()
 
 	var mesh := st.commit()
 	var mi := MeshInstance3D.new()
@@ -71,7 +75,10 @@ func _build() -> void:
 func _height(noise: FastNoiseLite, x: float, z: float) -> float:
 	return noise.get_noise_2d(x, z) * height_scale
 
-func _add_tri(st: SurfaceTool, a: Vector3, b: Vector3, c: Vector3) -> void:
+func _add_tri(st: SurfaceTool, a: Vector3, b: Vector3, c: Vector3, half: float, uv_scale: float) -> void:
+	st.set_uv(Vector2((a.x + half) * uv_scale, (a.z + half) * uv_scale))
 	st.add_vertex(a)
+	st.set_uv(Vector2((b.x + half) * uv_scale, (b.z + half) * uv_scale))
 	st.add_vertex(b)
+	st.set_uv(Vector2((c.x + half) * uv_scale, (c.z + half) * uv_scale))
 	st.add_vertex(c)