fix: stabilize border relocation for degenerate PCA axes #445

Overview

The profiling chain in autolens_profiling#113/#115 proved that NumPy FNNLS and JAX PDIP agree when given the same system, then isolated the native backend gap to ellipse_params_via_border_pca_from. For a near-isotropic traced border, the covariance eigenvalues differ only at floating-point scale, so NumPy and JAX choose different valid PCA eigenvectors. The helper then derives maximum ellipse extents along that arbitrary angle, turning eigenvector non-uniqueness into different relocated grids.

Measured on the full FitImaging pixelization fixture:

• raw traced source-grid error: exactly zero
• relocated source-grid error: up to 2.835e-1
• mapping-matrix error: up to 1.492e-1
• curvature-system error: up to 8.690e-3
• reconstruction error: up to 8.989e-3
• with border relocation disabled: curvature error 1.595e-16
• with a deterministic near-isotropic PCA axis: relocated-grid error exactly zero

Plan

• In ellipse_params_via_border_pca_from, compute the covariance eigenvalue gap relative to its scale.
• When that gap is no larger than sqrt(machine epsilon), select a deterministic axis-aligned frame before deriving ellipse extents.
• Retain the ordinary PCA major axis for non-degenerate covariance.
• Add NumPy unit coverage for the near-isotropic branch and the anisotropic control.
• Validate the JAX xp path through the downstream full-likelihood parity reproducer.

Boundary

Do not change NNLS algorithms, solver tolerances, public APIs, configuration, or behavior for non-degenerate PCA borders.