feat: Graph Motif Analyzer — detect and count subgraph patterns

Add MotifAnalyzer for network motif analysis:

Detected motifs:
- Triangles (3-cliques) with density and clustering coefficient
- Squares (4-cycles) via common-neighbor counting
- k-Stars (3/4/5-stars) via combinatorial degree counting
- Wedges (open triads) and open paths (P3)

Derived metrics:
- Global clustering coefficient (3 * triangles / wedges)
- Triangle density (triangles / C(n,3))
- Transitivity ratio (closed triads / total triads)
- Per-vertex motif participation counts
- Normalized motif fingerprint vector for cross-graph comparison
- Top participants ranking per motif type

Usage:
  MotifAnalyzer ma = new MotifAnalyzer(graph).compute();
  System.out.println(ma.getTriangleCount());
  System.out.println(ma.getClusteringCoefficient());
  System.out.println(ma.getSummary());

41 new tests, all pass.

Author: sauravbhattacharya001