Add xp-API dense kernels + iterative LM engine (potential correction phase 4)

Refactors the jax_ops kernels of Cao et al. 2025's potential_correction
package onto the PyAuto xp API: al.pc.dense_util writes each dense
kernel once (xp=np default, jax.numpy for accelerator execution) —
evidence variants incl. the fixed-curvature Cholesky fast path, and the
Levenberg-Marquardt kernels. Adds al.pc.IterFitDpsiSrcImaging, the
iterative joint source+dpsi LM engine (re-ray-traces through the
corrected lens via the phase-2 InputPotential each accepted step;
optional gauge constraints via a KKT step) and its
IterDpsiSrcInvAnalysis. Fixes an upstream NaN bug in split_cross_from
(percentile over unbounded-cell sentinels).

Phase 4 of #618; stacked on #621.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 2 people

autolens/potential_correction/__init__.py

@@ -26,3 +26,6 @@
 from autolens.potential_correction.fit import FitDpsiSrcImaging
 from autolens.potential_correction.analysis import DpsiInvAnalysis
 from autolens.potential_correction.analysis import DpsiSrcInvAnalysis
+from autolens.potential_correction import dense_util
+from autolens.potential_correction.iterative import IterFitDpsiSrcImaging
+from autolens.potential_correction.iterative import IterDpsiSrcInvAnalysis