Add missing 'import matplotlib as mpl' for the i18n font config

The resync kept the CJK font-configuration block but dropped the mpl
import it depends on - a NameError at execution that non-strict builds
swallow. Found sweeping the wave for the pattern behind zh-cn#97's
December fixes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/likelihood_var.md

@@ -67,6 +67,7 @@ translation:
 import numpy as np
 import matplotlib.pyplot as plt
 FONTPATH = "fonts/SourceHanSerifSC-SemiBold.otf"
+import matplotlib as mpl
 mpl.font_manager.fontManager.addfont(FONTPATH)
 plt.rcParams['font.family'] = ['Source Han Serif SC']