Closes #564

Summary

• Create docs/CROSS_REFERENCE_GUIDE.md as the canonical cross-reference convention for the ecosystem
• Add intra-project cross-references (blockquote format) to ARCHITECTURE.md Overview and References sections
• Add Mermaid ecosystem dependency diagram to both README.md and ARCHITECTURE.md
• Add inter-project ecosystem references linking to all downstream project documentation

Test Plan

• Verify all intra-project links resolve correctly
• Verify Mermaid diagram renders properly on GitHub
• Verify inter-project GitHub URLs point to valid documents