Standardize all raster images to AVIF format

Convert all 48 JPEG/PNG raster images across book/ and slides/ to AVIF
(quality 65), reducing total image size from 28 MB to 15 MB (~48% savings).
Update all MyST figure references from .jpg/.png to .avif. Add avif2png
Makefile targets that auto-generate PNG for XeLaTeX builds (which cannot
read AVIF natively). Gitignore generated PNGs in both book/ and slides/.

Encode AVIF as the universal image format standard in CLAUDE.md (new
"Image Format" section) and planning/course_development.md (new
"Technical Standards" section).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timlichtenberg

.gitignore

@@ -5,6 +5,10 @@ content/course2025
 # Jupyter Book build artifacts
 book/_build/
 
+# Generated from AVIF at build time (see Makefile avif2png target)
+book/**/figures/*.png
+book/**/figures/*.jpg
+
 # Byte-compiled / optimized / DLL files
 __pycache__/
 *.py[codz]

CLAUDE.md

@@ -1,16 +1,29 @@
-.PHONY: html pdf pdflatex slides clean
+.PHONY: html pdf pdflatex slides clean avif2png
 
 html:
 	jupyter-book build book/
 
 pdf:
 	jupyter-book build book/ --builder pdfhtml
 
-pdflatex:
-	jupyter-book build book/ --builder pdflatex
+# Generate LaTeX, convert AVIF→PNG for XeLaTeX, then compile
+pdflatex: avif2png
+	jupyter-book build book/ --builder latex
+	@find book/_build/latex/ -name '*.avif' -exec sh -c \
+		'magick "$$1" "$${1%.avif}.png"' _ {} \;
+	@sed -i '' 's/\.avif/.png/g' book/_build/latex/*.tex
+	cd book/_build/latex && make
+
+# Convert AVIF→PNG beside originals for LaTeX builds
+avif2png:
+	@echo "Converting AVIF → PNG for LaTeX..."
+	@find book/ -name '*.avif' ! -path '*/_build/*' -exec sh -c \
+		'magick "$$1" "$${1%.avif}.png"' _ {} \;
 
 slides:
 	$(MAKE) -C slides
 
 clean:
 	jupyter-book clean book/
+	@find book/ -name '*.avif' ! -path '*/_build/*' \
+		-exec sh -c 'rm -f "$${1%.avif}.png"' _ {} \;