robscale 0.2.1

• Performance: Refined runtime SIMD dispatch for qn() and sn() kernels,
  achieving optimal vectorization across x86_64 (AVX2/FMA) and ARM64 (NEON).
• New feature: scale_robust() provides a unified dispatcher that automatically
  selects between a variance-weighted ensemble of 7 estimators (for small
  samples) and the Gini Mean Difference (GMD) for larger samples (n >= 20).
• New estimators: Added gmd(), iqr_scaled(), mad_scaled(), and sd_c4()
  to the public API.
• Confidence Intervals: Added ci = TRUE to all scale estimators, providing
  analytical intervals (GMD, MAD, Sn, Qn, bias-corrected SD) or bootstrap-based
  intervals (ensemble).
• Performance: Replaced stats::mad() and stats::IQR() with optimized C++
  implementations (mad_scaled(), iqr_scaled()) using O(n) selection via the
  pdqselect algorithm.
• Documentation: Expanded README with detailed benchmarking vs. robustbase,
  Hmisc, GiniDistance, and collapse.
• Citations: Consolidated and updated all package citations in inst/CITATION
  and documentation.