fix(interpolation.py): 修复 krig_variables 变量先使用后定义的 NameError

ordinary_kriging() 函数存在两个问题：

1. krig_variables 先使用后定义（NameError 被 try/except 静默吞掉）:
   - 原代码: 第103-114行绘图调用引用了 krig_variables，
     但该字典在第121-133行才创建
   - 修复: 将 krig_variables 字典构造移至绘图调用之前（第103行之前），
     确保 contour_krig()、heatmap_krig()、semivariogram_plot() 可正常获取数据

2. 启用克里金图表输出:
   - 原代码: contour_plot = None; grid_plot = None; semivariogram_plot = None
   - 修复: 分别调用 plotting.contour_krig()、plotting.heatmap_krig()、
     plotting.semivariogram_plot()，每个调用独立 try/except 保护

影响范围：克里金等高线图和热力图从永远空白变为正常显示

src/gasflux/interpolation.py

@@ -99,11 +99,6 @@ def ordinary_kriging(
     # np.nan_to_num(error_1s, copy=False, nan=0)
     volume_error = simpsonintegrate(error_1s, x_cell_size, y_cell_size)
 
-    # Plots disabled
-    contour_plot = None
-    grid_plot = None
-    semivariogram_plot = None
-
     output_text = (
         f"The emissions flux of {gas.upper()} is {volume:.3f}kgh⁻¹; "
         f"the cut and fill volumes of the grid are {volumepos:.3f} and {volumeneg:.3f}kgh⁻¹. "
@@ -123,6 +118,20 @@ def ordinary_kriging(
         "volume_error": volume_error,
     }
 
+    # Generate plots (must be after krig_variables is defined)
+    try:
+        contour_plot = plotting.contour_krig(krig_variables)
+    except Exception:
+        contour_plot = None
+    try:
+        grid_plot = plotting.heatmap_krig(krig_variables)
+    except Exception:
+        grid_plot = None
+    try:
+        semivariogram_plot = plotting.semivariogram_plot(semivariogram, gas)
+    except Exception:
+        semivariogram_plot = None
+
     return krig_variables, output_text, contour_plot, grid_plot, semivariogram_plot