Fix tofu in Chinese figure labels, and give font.family a fallback (#206)

* Restore the CJK font block in three lectures (#205)

These three lectures set Chinese text on their figures but never register a
CJK font, so matplotlib falls back to DejaVu Sans — which has no CJK coverage
— and every Chinese axis label, title and legend entry renders as blank boxes
("tofu"). Nothing errors and a valid PNG is written, so the build stays green
and the defect only shows up in the published output.

Affected labels include 密度 / KL散度 / JS散度 / 切尔诺夫熵 in
divergence_measures.md, 收益率 / 日期 / 时间 / 对数秩 / 对数规模 in
kesten_processes.md, and the subplot titles 价值函数 / $s$策略 / $\phi$策略
in jv.md.

Adds this edition's standard block to the first code cell that imports
matplotlib in each file, immediately after `import matplotlib.pyplot as plt`
so it runs before any figure is drawn:

    import matplotlib as mpl
    FONTPATH = "fonts/SourceHanSerifSC-SemiBold.otf"
    mpl.font_manager.fontManager.addfont(FONTPATH)
    plt.rcParams['font.family'] = ['Source Han Serif SC']

Matches the convention already used by the other 112 lectures here (see
aiyagari.md); the path resolves against lectures/, where
fonts/SourceHanSerifSC-SemiBold.otf is checked in. Same class of gap as the
five lectures fixed in #196 — nothing else in these files is touched.

See #205

* Add a DejaVu fallback to font.family, and register the font in two more lectures

Setting font.family to a single entry removes matplotlib's fallback, so any
character the CJK font lacks renders as a blank box. Source Han Serif SC follows
Adobe-GB1, which omits subscript digits (U+2080-2083), the vertical ellipsis
(U+22EE), and the lunate/symbol Greek forms (U+03F5 epsilon, U+03D5 phi) — all
of which these lectures use in figure labels.

Measured on matplotlib 3.11.1 with this repo's own font, rendering a label
containing 密钥拆分树, key₀, ⋮, ϵ and ϕ:

    font.family = ['Source Han Serif SC']                  -> 4 missing-glyph warnings
    font.family = ['Source Han Serif SC', 'DejaVu Sans']   -> 0

Two lectures already carried the block and were silently losing characters to
it: career.md sets ylabel='ϵ' (and label='ϵ'), affine_risk_prices.md builds
label=f"... (r₁ = ...)". Both are fixed by the fallback alone.

This deviates from the single-entry dialect used elsewhere in the edition. That
is deliberate: the dialect is missing a fallback, and adding one is strictly an
improvement everywhere it appears.

Author: mmcky

lectures/affine_risk_prices.md

@@ -568,8 +568,7 @@ mystnb:
 import matplotlib as mpl  # i18n
 FONTPATH = "fonts/SourceHanSerifSC-SemiBold.otf"  # i18n
 mpl.font_manager.fontManager.addfont(FONTPATH)  # i18n
-mpl.rcParams['font.family'] = ['Source Han Serif SC']  # i18n
-
+mpl.rcParams['font.family'] = ['Source Han Serif SC', 'DejaVu Sans']  # i18n
 n_max_1f = 60
 maturities_1f = np.arange(1, n_max_1f + 1)
 

lectures/career.md

@@ -64,7 +64,7 @@ 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']
+plt.rcParams['font.family'] = ['Source Han Serif SC', 'DejaVu Sans']
 
 from matplotlib import cm
 from mpl_toolkits.mplot3d.axes3d import Axes3D

lectures/divergence_measures.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', 'DejaVu Sans']
 import numpy as np
 from numba import vectorize, jit
 from math import gamma

lectures/jv.md

@@ -53,6 +53,10 @@ translation:
 from typing import NamedTuple
 
 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', 'DejaVu Sans']
 import scipy.stats as stats
 import jax
 import jax.numpy as jnp

lectures/kesten_processes.md

@@ -73,6 +73,10 @@ 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', 'DejaVu Sans']
 import numpy as np
 import quantecon as qe
 import yfinance as yf