Simulation Boundary Research (SBR), Volume III: Cross-Domain Synthesis and Constraint-Architecture Assessment

Volume III of Simulation Boundary Research (SBR) examines whether the six heterogeneous empirical records developed in Volume II support any defensible cross-domain synthesis once their native measurands, measurement models, uncertainty semantics, representation dependence, conventional explanations, dependence structure and outcome exposure are preserved. The core analysis uses claim-specific evidence admission, comparability assessment, typed dependence, serious separate-domain comparators and pre-fit identification requirements. Four bounded claim families are examined. RP-II and Unitarity-Limited Scattering retain only structural analogy at the relevant cross-domain level; Relativistic Propagation and Operational Quantum Speed Limits support a narrow point-level theory-bound order relation but not the stronger proposed common-set construction; LIGO and Lattice String Breaking retain structural analogy only; and the programme-wide assessment finds no single empirically constraining common object spanning all six domains. No common map, common likelihood, common fit, common parameter, identified set or prospective-validation programme is therefore warranted by the current evidence. A separately governed supplementary study asks whether an independently motivated architecture of heterogeneous compatibility constraints can add cross-domain structure without requiring a universal common parameter or reopening the frozen core analysis. The study defines the scientific object A = (V, X, C, G, P) and evaluates twenty candidate cross-object constructions. None survives as an eligible substantive cross-object constraint: C is empty. Descriptive relations and methodological or semantic guardrails remain useful, but they do not reduce the joint physical state space beyond the separate-domain baseline. Higher-order constraint-architecture status is therefore not achieved, and no subsequent fitting, locking or prospective validation is performed. The principal contribution of Volume III is consequently methodological and constraint-setting. It shows how cross-domain synthesis can terminate scientifically when heterogeneous evidence does not support a defensible shared quantitative object, rather than manufacturing comparability through post hoc normalization or aggregation. The result does not establish or refute simulation ontology, a common generator, a universal computational boundary or any external architecture. This volume marks the scientific endpoint of the current SBR programme at SBR III. The separate programme decision not to undertake SBR IV under the current programme is a lifecycle decision and is not presented as a scientific consequence of the negative synthesis result. This work is independent private research conducted with extensive AI-assisted research support under the final scientific and authorial responsibility of Thomas Johannes Schultheiss.