Add flyby geometry, cubic spline method, and sample count sweep

[emilymfoley]

Added flyby geometry, cubic spline method, and sample count sweep.

• Wired up flyby and tetrahedron_like geometries that were defined in geometries.py but unreachable via generate_sampling_points
• Added CubicSplineMethod (Clough-Tocher 2D) following the existing base class pattern
• Added scripts/run_sample_count_sweep.py comparing RBF, linear, nearest, and cubic spline across n = 10, 25, 50, 100, 200 random samples
• Added tests for all geometry functions including flyby in 2D and 3D

Found:

• RBF covers 100% of the domain at all sample counts and reaches rel L2 = 0.077 at n=200
• Cubic spline has lower error than RBF within the convex hull, but only covers ~84% of the domain at n=200 — same coverage limitation as linear interpolation
• Nearest neighbor is consistently the weakest method
• Linear and cubic spline have identical domain coverage at every sample count since both are triangulation-based