ATMAN v5.0 Paper XI: Cross-validation of the Erebo Ecliptic Longitude in the Full Three-Perturber Background

Paper XI of the ATMAN v5.0 series. The ecliptic longitude sweep of Paper II for Erebo (7M⊕, 600au) is repeated with Solution B and P3 present as fixed background perturbers, to test whether the Paper II best-fit of λErebo=156° shifts when the full three-perturber potential is active. The full 360° coarse sweep (Δλ=10°) plus fine refinement (Δλ=2°) reveals that all longitude trials produce essentially identical combined scores (score standard deviation=2.9×10⁻⁶, range=7.5×10⁻⁶ over 360°). The Paper II result λErebo=156°±4° remains valid and uncontaminated. The three-perturber system is internally consistent: no perturber displaces another's optimal longitude. SHA-256: 06271566c09e9214201dba1c5fab3342678dfe1bfe81ed0056d0d72229d68c21. Paper XI of the ATMAN v5.0 series.