perf: barycentric lagrange interpolation for faster reconstruction

[lonerapier]

Motivation

Current reconstruction uses standard Lagrange interpolation, which recomputes coefficients every time.
Barycentric form dramatically improves performance and numerical stability, especially for repeated reconstruction with fixed x-coordinates.

Tasks

• Implement barycentric weight computation.
• Add reconstruction method using precomputed weights.
• Add criterion benchmarks comparing old/new methods.
• Update docs explaining trade-offs and usage.

References

• Berrut & Trefethen, “Barycentric Lagrange Interpolation” - SIAM Review (2004)