Fix Chapter 3 SVG arrow routing

[restful3]

Summary

• audit all 8 Chapter 3 SVGs and all 34 directional arrow paths
• route Figure 4 loop arrows to the correct box side centers
• standardize visible arrowheads and restore clear shafts for short connectors
• separate validation return anchors and clarify intentionally undirected clinical-query lines
• resolve label overflow and align repeated diagram elements consistently

Validation

• xmllint --noout for all 8 SVGs
• python3 agent-support/scripts/build-index.py --check
• python3 agent-support/scripts/validate-site.py --check-materials
• all 27 slides at 1280×720, 1366×768, and 390×844 with no clipping or overflow
• report at desktop/mobile widths; all 8 images loaded and lightbox verified
• Claude browser/DOM review: PASS; 34/34 arrow endpoints on intended boundaries, no head-only arrows