Add amsfonts to LaTeX preamble so \mathbb renders under usetex

The upstream rewrite introduced \mathbb{E}[...] notation in plot titles
and labels; the CJK-adapted preamble carried only CJKutf8+amsmath, so
under text.usetex=True the \mathbb control sequence was undefined and
fig.tight_layout() raised RuntimeError (latex failed). Upstream's own
preamble is amsmath,amsfonts; this restores amsfonts alongside the
zh-specific CJKutf8 adaptation.

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

Author: mmcky

lectures/kalman_2.md

@@ -79,6 +79,7 @@ mpl.rcParams.update({
     'text.latex.preamble': r'''
         \usepackage{{CJKutf8}}
         \usepackage{{amsmath}}
+        \usepackage{{amsfonts}}
     '''
 })