Implement B-splines

[jlogan03]

0.11.1 2026-06-13

Changed

• Extract Hermite node weights from eval loop for cubic methods
  • 3x asymptotic speedup for large data; modest improvement in latency
• Use inlined helper function for FMA feature switching

Added

• Add B-spline methods for regular and rectilinear grids
  • Tridiagonal coeff solve using preallocated scratch space, optionally parallelized
  • Use same BCs as Hermite methods (zero third derivative) for reduced ringing and graceful transition to linearized extrapolation