Add layered diagram system with simple/detailed toggle

Implement progressive disclosure for Mermaid diagrams:
- Simple mode (default): shows simplified overview with fewer nodes
- Detailed mode: shows full diagram (existing behavior)
- Toggle button visible on all diagrams
- Click node in Simple mode to see focused sub-diagram
- Back button to return to overview from sub-diagram
- Collapsible color legend on first diagram per page
- One-time "click nodes for details" hint tooltip
- New mermaidSimplifier utility for parsing and simplifying
  flowcharts, sequence diagrams, and class diagrams

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

Author: 2 people