feat: add Graph Comparison Tool - side-by-side structural similarity analysis

Interactive tool to compare two graphs with:
- Side-by-side Canvas rendering with drag-to-move nodes
- 8 graph presets (Petersen, K4, K3,3, Cube, Cycle-6, Star-5, Path-5, Random)
- 12 structural metrics comparison (nodes, edges, density, degree stats, diameter, radius, components, clustering coefficient, bipartiteness, chromatic number)
- 3 similarity algorithms: degree sequence, spectral (power iteration eigenvalues), edit distance estimate
- Weighted overall similarity score
- Degree distribution bar chart comparison
- JSON export of results

Usage: Open docs/graph-compare.html, load presets or enter edge lists, click Compare

Author: sauravbhattacharya001