Implement complete graph and statistics modules following spec v1.1.0 with Toyota Way principles.

Scope

Statistics Module

• Quantiles with QuickSelect O(n) optimization
• R-7 interpolation method
• Percentiles, five-number summary, IQR
• Histogram methods: FreedmanDiaconis, Sturges, Scott, SquareRoot, Bayesian

Graph Module

• CSR (Compressed Sparse Row) representation
• Degree centrality
• PageRank with Kahan summation
• Parallel betweenness centrality with Rayon

Specifications

• docs/specifications/graph-traditional-descriptive-statistics-spec.md v1.1.0
• docs/specifications/toyota-way-review-summary.md

Quality Gates

• Tests: 33+ new tests
• Clippy: Clean with -D warnings
• TDG: A grade (90+)
• Coverage: >85%