Physical Stellar-Burden and Domain-Boundary Response in the ECSM Framework: A KiDS-1000 PneE Projection Test Using KiDS DR4 LePhare Stellar Masses

This paper presents a KiDS-1000 PneE projection test of physical stellar-burden and domain-boundary response terms in the ECSM framework. Using the KiDS DR4 bright-galaxy LePhare stellar-mass catalogue, the analysis constructs physical stellar-burden drivers by lens bin and combines them with lens/source projection geometry, domain-boundary-gradient terms, and finite-response kernels. The result is a physically improved projection-bridge test rather than a final first-principles baryonic weak-lensing prediction. ECSM response branches strongly outperform smooth and null controls in full-data and held-out source-bin comparisons, while held-out lens-bin validation identifies the remaining bottleneck: the need for the exact foreground lens catalogue with matched stellar mass, luminosity, size, or baryonic proxy information.