Add Ch1–Ch5 solutions and solution images

[davidaustinm]

Summary

• Add solutions to all Ch1–Ch5 activities, previews, and exercises (sections 1.1–5.5)
• Ch1: solutions drawn from Lindsay Rocks' LaTeX source; gaps filled using Steve's MTH124 materials
• Ch2–Ch5: solutions drawn from Lindsay Rocks' LaTeX source
• Add ~45 solution images (Lindsay's Desmos graphs and sketches) to assets/solution-images/ for Ch1 activities and exercises; images inserted into <solution> elements where appropriate
• Minor publication/publication.ptx update

Tagging @Tanaquil18 for review.

Test plan

• All modified XML files validate (lxml parse check)
• Build with pretext build web and verify solution content renders correctly
• Spot-check a sample of solutions and images in the built output

🤖 Generated with Claude Code

[davidaustinm]

The source builds with pretext build and a spot-check of solutions and images looks good.