fix(vol1): two CI-surfaced PDF build errors

1. nn_architectures: remove \renewcommand{\arraystretch}{1.1} from
   5 instances inside $$...$$ math blocks (display math). Pandoc
   was macro-expanding \arraystretch (set to 1.25 by earlier chapter
   ml_systems' raw-LaTeX \begingroup blocks), producing the broken
   \renewcommand{1.25}{1.1} in the rendered LaTeX. The 6th
   \renewcommand in this file (inside a {=latex} block at line 5042)
   is safe and untouched.

2. header-includes.tex: add \providecommand{\tcbbreak}{} fallback.
   PR #1789 (callout-audit) introduced \tcbbreak in 3 chapter QMDs
   to force tcolorbox page-breaks, but the macro isn't defined by
   the local foldbox extension. The fallback no-ops cleanly when
   tcolorbox doesn't provide it.

Surfaced by CI Linux Vol I PDF build (run 26151749093, job
76922049346). Both errors reproduce locally and are fixed by these
two changes. Vol I PDF builds cleanly post-fix.

Author: profvjreddi

book/quarto/contents/vol1/nn_architectures/nn_architectures.qmd

@@ -1640,7 +1640,7 @@ The equivariant layers preserve where features occur, enabling the network to le
 ::: {#exmp-nn-architectures-equivariance-feature-detection .callout-example title="Equivariance: Feature detection"}
 
 Consider a $7{\times}7$ image with a vertical edge at column 3:
-$$\renewcommand{\arraystretch}{1.1}
+$$
 \mathbf{x} = \begin{bmatrix}
 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
@@ -1653,7 +1653,7 @@ $$\renewcommand{\arraystretch}{1.1}
 $$
 
 Vertical edge detector filter:
-$$\renewcommand{\arraystretch}{1.1}
+$$
 \mathbf{w} = \begin{bmatrix}
 -1 & 0 & 1 \\
 -1 & 0 & 1 \\
@@ -1664,7 +1664,7 @@ $$
 **Convolving original image**:
 
 Output feature map shows positive activation where the filter transitions from dark to bright (left side of edge) and negative activation where it transitions from bright to dark (right side):
-$$\renewcommand{\arraystretch}{1.1}
+$$
 f(\mathbf{x}) = \begin{bmatrix}
 3 & 0 & -3 & 0 & 0 \\
 3 & 0 & -3 & 0 & 0 \\
@@ -1675,7 +1675,7 @@ f(\mathbf{x}) = \begin{bmatrix}
 $$
 
 **Shifted input** (edge moved to column 5):
-$$\renewcommand{\arraystretch}{1.1}
+$$
 T_2 \mathbf{x} = \begin{bmatrix}
 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
@@ -1688,7 +1688,7 @@ T_2 \mathbf{x} = \begin{bmatrix}
 $$
 
 **Convolving shifted image**:
-$$\renewcommand{\arraystretch}{1.1}
+$$
 f(T_2 \mathbf{x}) = \begin{bmatrix}
 0 & 0 & 3 & 0 & -3 \\
 0 & 0 & 3 & 0 & -3 \\

book/quarto/tex/header-includes.tex

@@ -2052,3 +2052,8 @@
 
 \makeatother
 
+
+% Fallback for \tcbbreak (used by callout-audit PR #1789 to force tcolorbox
+% page-breaks). If a runtime tcolorbox version doesn't define it, no-op so
+% the PDF builds cleanly. See dev commit fee17cc2c0 and downstream merges.
+\providecommand{\tcbbreak}{}