Fix navigation merge errors, fixes #56786

- improved `detail/sample_max_error` default value
- improved floating point precision handling in cell key calculations
- improved `merge error` error message
- exposed `cell_height` of `nav_map` to the `Navigation`
- fixed cell key `y` calculation

modules/navigation/nav_map.h

@@ -50,8 +50,9 @@ class NavMap : public NavRid {
 	Vector3 up;
 
 	/// To find the polygons edges the vertices are displaced in a grid where
-	/// each cell has the following cell_size.
+	/// each cell has the following cell_size and cell_height.
 	real_t cell_size;
+	real_t cell_height;
 
 	/// This value is used to detect the near edges to connect.
 	real_t edge_connection_margin;
@@ -95,6 +96,11 @@ public:
 		return cell_size;
 	}
 
+	void set_cell_height(float p_cell_height);
+	float get_cell_height() const {
+		return cell_height;
+	}
+
 	void set_edge_connection_margin(float p_edge_connection_margin);
 	float get_edge_connection_margin() const {
 		return edge_connection_margin;