Bridge Factorization and the Limits of Metaphysics-to-Physics Derivation

A metaphysics-to-physics bridge should be treated as a sequence of distinct warrant-bearing transformations rather than as a single derivation. This paper develops a Quantitative Bridge Audit for reconstructions in which a qualitatively thin source is enriched into quantitative physics. Source fidelity, numerical representation, physical typing, quantitative selection, realization, and empirical exposure answer different questions; success at one layer does not by itself establish another. An invariance-or-selector requirement tests whether a number can be attributed to the source, while a Type–Law Separation Result shows that a legitimate target-domain stress-energy interpretation can coexist with underdetermination of the quantitative law. A worked dark-energy case makes the method reflexive. A reception–expenditure normalization yields wₛ = −(1 − s²)/(1 + s²) exactly within the adopted representation, but not as a source-invariant equation of state. A simple linear-field/quadratic-potential canonical realization fails analytically, whereas a broader canonical reconstruction can realize a granted smooth non-phantom history under the stated positivity, conservation, and canonical-scalar conditions. The residual result is methodological: realizability, physical typing, numerical selection, source derivation, and evidential support are distinct achievements, and empirical success can be credited upstream only through an explicit account of the dependency and confirmation structure