Mass as Decompression in a Metric-Locked Framework: Endpoint Constraints on a Decompression-Defect Matter Ontology

This paper develops an endpoint-first, metric-locked formulation of the Mass as Decompression hypothesis. Matter is interpreted as localized decompression defects within an underlying compressed state, with rest mass arising from localized defect energy, while ordinary long-range gravity remains a universal metric response to total stress-energy. Unlike earlier scalar-pressure analogs, the active branch does not identify the compression/decompression field with gravity. A fixed two-body derivative spin-current/axial-gradient tensor sector, supplemented by finite-range triple-overlap saturation and an alpha-normal-ordered open-shell residual, is assessed through helium-4 and A = 3 transfer diagnostics. Representative results are Eα approximately −28.30 MeV, Et approximately −8.48 MeV, and E³He approximately −7.78 MeV at R₃ = 3.0 fm and W₃ = 35.006 MeV. These are endpoint scouts, not ab initio nuclear fits. The gravitational embedding is constrained through total stress-energy sourcing, Newtonian recovery, composite-body free fall, post-Newtonian scalar leakage, Cassini bounds, MICROSCOPE composition-dependence limits, and quadratic stability conditions. The surviving branch is metric locked: decompression may organize internal rest mass only if ordinary long-range gravity remains universal and metric. The result is a constrained scaffold for further theoretical investigation, not a complete replacement for General Relativity or a complete nuclear theory.