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The Two-Face Threshold: Inward Realization, Selective Interface, and Outward Participation v0.1

A bounded unit has an inward realization and an outward participation only relative to a declared boundary. Between them lies a selective interface, not an identity. This paper represents a threshold by an inward space X, environment E, boundary, outward description L, and map ell:X→L. It proves that a noninjective interface cannot support unique inward reconstruction and gives a finite fiber example. FRC 830 strong morphisms then provide a stricter commuting-square test for proposed transformations. The result constrains the FRC μ lens without deriving the 830 mathematics from it.