Discrimination ratio = mean_fault_rate / (mean_healthy_rate + 0.01)
aggregated across all detectors in each axis, across all 12 fixtures.
Higher = axis fires more selectively on fault windows.
A ratio near 1.0 indicates the axis fires equally on healthy/fault.
A ratio near 0.0 indicates the axis is dormant on the current surface.
Source: Phase ζ.6 audit harness (`src/audit/axis_discrimination.rs`).