feat: add Graph Fairness Audit Engine - autonomous network equity analyzer

7 engines: degree equity (Gini on degree distribution + Lorenz curve), centrality
equity (betweenness centrality Gini via BFS), access equity (average shortest-path
inequality), community representation (label propagation + size Gini), resource
proximity (multi-source BFS desert detection), fairness intervention planner
(greedy edge-addition recommendations), insight generator.

Features:
- Composite fairness score 0-100 with 5 tiers (EXEMPLARY/FAIR/MODERATE/INEQUITABLE/CRITICAL)
- Lorenz curve visualization for degree distribution
- Resource desert detection (nodes far from high-degree hubs)
- Autonomous intervention planning with ranked edge additions
- Interactive HTML dashboard with dark theme, Lorenz curve chart, tier badges
- Text report with spark bars and ranked insights
- Builder-style configuration (resource count, max interventions, seed)
- Handles edge cases: empty graph, single node, disconnected components

76 tests covering all engines, boundary conditions, immutability, and exports.

Author: sauravbhattacharya001