Fixes pgfplots dimension overflow in data_selection chapter

- Fixes fig-amortization-comparison: scales Y-axis values from 12000 to 12
  to avoid LaTeX dimension limit (~16383pt)
- Fixes fig-compute-optimal-frontier: replaces problematic \fill...plot
  with proper \addplot[fill=...] \closedcycle for log-scale coordinates
- Updates figure reference text to use @fig-selection-inequality
- Adds ch_data_selection.py calculation module
- Updates viz.py with new plot functions
- Various chapter updates across vol1 and vol2

.pre-commit-config.yaml

@@ -176,6 +176,14 @@ repos:
         pass_filenames: true
         files: ^book/quarto/contents/.*\.qmd$
 
+      - id: book-check-four-colon-space
+        name: "Book: Check for malformed nested div fences (::::)"
+        entry: >-
+          bash -c 'for f in "$@"; do grep -nE "^::::[ ]+[^{ ]" "$f" && echo "  ^^^ ERROR in $f: nested div fence (::::) has trailing text. The :::: must be on its own line to close a nested div, or use '\''{.class}'\'' to start one." && exit 1; done; exit 0' --
+        language: system
+        pass_filenames: true
+        files: ^book/quarto/contents/.*\.qmd$
+
       - id: book-check-figure-completeness
         name: "Book: Check figures have captions and alt-text"
         entry: python book/tools/scripts/content/check_figure_completeness.py -d book/quarto/contents/ --strict --quiet