Selection Fronts and Boundary-Layer Physics at the Admissibility Threshold

An admissibility boundary is the place where at least one declared control condition loses positive reserve. That static fact does not by itself produce a boundary layer, metastability, a large-deviation law, a measurement outcome, or a new physical phase. Those conclusions require a specified dynamical model. This paper gives a precise conditional meaning to the term selection front. The input is a finite vector of normalized signed reserve rows, including the rows needed for spectral separation, projector control, well-posedness, descent, coherent stability, truncation, and numerical certification. The exact boundary is distinguished from a conservative numerical event guard. At one regular active row the boundary is a smooth facet; at transverse row ties it is locally an orthant corner. A dynamical front additionally requires a drift, diffusion, initial law, stopping rule, interior return set, observables, and a post-exit stop, chart-transition, reset map, or reset kernel. The central calculation is an exactly solvable normal diffusion on an interval. Its guard-hitting committor and mean exit time are derived in closed form. Drift away from the guard gives an exponential boundary layer of thickness proportional to noise divided by drift; zero drift gives a linear committor and inverse-noise exit time; drift toward the guard gives neither behavior. Thus no universal sigmoid, metastable law, or first-passage scaling follows from admissibility alone. Eyring–Kramers and Freidlin–Wentzell asymptotics remain available when a small-noise family, attracting basin, quasipotential barrier, and regularity hypotheses are independently supplied. The result is a reproducible local theory of first exit and a strict application contract. It does not derive vacuum selection, black-hole dynamics, cosmological measures, quantum collapse, or limits on scalable coherence.