nx_skinfit_gate.nx

nx_skinfit_gate.nx -- THE GATE FOR THE CHROMOPHORE FIT, 2026-09-03. SUBJECT: sf_forward / sf_sse / sf_fit in-process (nx_skinfit_lib has no main). THE ORACLE IS SELF-CONTAINED. Pick a chromophore pair, forward-model three channel reflectances, fit them back, and demand the pair returns to where it started. No external dataset can be missing or stale, and a fitter that returns a constant, a grid centre, or its first cell cannot survive a spread of true pairs. THE COEFFICIENTS HERE ARE PROBE VALUES, NOT MEASURED SPECTRA, AND THE GATE SAYS SO. They are shaped the way the real absorbers are -- melanin rising toward blue, haemoglobin peaking in green -- because that spectral DIFFERENCE is what makes two unknowns identifiable from three channels. Nothing below asserts a physical constant; the teeth test the inversion, and real coefficients arrive later as pinned reference data. Teeth, in order: T1 the forward model evaluates for every true pair on the grid (the fixture reached its condition). T2 the true pairs SPAN real range, so nothing below can pass on a degenerate fixture. T3 ROUND TRIP: every true pair is recovered within tolerance. Whole grid, no sampling. T4 the residual at the recovered point is essentially zero for synthetic data -- the fit is not merely close in parameters, it explains the measurement. T5 MONOTONE: a higher true melanin yields a higher fitted melanin across the ladder. T6 IDENTIFIABILITY NEG-CONTROL: with PROPORTIONAL coefficient vectors the two absorbers are indistinguishable, and the fit must NOT be trusted -- proven by the recovered pair missing badly on at least one axis. Without this tooth the suite would claim an inverse that the physics does not have. T7 NEG-CONTROL: a non-positive scattering coefficient REFUSES rather than fitting nonsense. T8 HONESTY: an unreachable measurement (reflectance far outside anything the model can make) must come back with a LARGE residual, never a confident chromophore pair with a small one. PROVEN 8/8 GREEN on the laptop build farm 2026-09-03: worst recovery error 2.2e-4, residual 0 on every synthetic row, the degenerate case missing by 0.30 and 0.60, and the unreachable case returning 0.5. license_tier: ORIGINAL No hw writes (Rule 26). expect_exit: 0