Add Vertex Cover Analyzer with 2-approx, greedy, exact, weighted algorithms, kernel reduction, and LP bounds

Features:
- 2-approximation vertex cover (edge-matching, guaranteed ≤ 2x optimal)
- Greedy vertex cover (max uncovered degree heuristic)
- Exact minimum vertex cover (brute-force for ≤ 20 vertices)
- Weighted vertex cover (min weight-to-degree ratio greedy)
- Cover verification and uncovered edge detection
- Per-vertex coverage contribution analysis
- Cover number bounds (matching + edge-based lower bounds)
- LP relaxation bound (half-vertex fractional cover)
- Kernel reduction (degree-0/1 rules, universal vertex rule)
- Full consolidated report

Usage: new VertexCoverAnalyzer(graph).fullReport()
73 tests included.

Author: sauravbhattacharya001