feat: Graph Merge Workshop - interactive set operations tool

Add interactive tool for combining two graphs using Union, Intersection,
Difference, and Symmetric Difference operations with:
- Dual canvas editors with click-to-add nodes/edges and drag support
- 6 graph presets per panel (Triangle, Square, K5, Path, Cycle, Petersen)
- Random graph generator and adjacency list text input
- Force-directed layout with staggered entrance animation
- Color-coded results (blue=G1 only, green=G2 only, purple=both)
- Live stats: node count, edge count, components, density
- Proactive insights analyzing structural overlap and suggesting next ops
- JSON adjacency list export to clipboard

Usage: Open docs/graph-merge.html, load or draw graphs, click an operation.

Author: sauravbhattacharya001