Restore CJK font configuration dropped by the resync

The resynced file keeps Chinese plot labels but reverted the import
cell to the source's exact form, losing the Source Han Serif setup -
Chinese in figures would render as missing glyphs. Residual #107
class found in the merge review; applied wave-wide.

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

Author: mmcky

lectures/ar1_turningpts.md

@@ -63,6 +63,10 @@ translation:
 
 ```{code-cell} ipython3
 import matplotlib.pyplot as plt
+import matplotlib as mpl
+FONTPATH = "fonts/SourceHanSerifSC-SemiBold.otf"
+mpl.font_manager.fontManager.addfont(FONTPATH)
+plt.rcParams['font.family'] = ['Source Han Serif SC']
 import seaborn as sns
 from typing import NamedTuple