Exemption taxonomy: unpopulated arms and the population census

The sealed Exemption taxonomy (graphitron test sources) carries an arm no registry row constructs: HarnessSingleCatalog has zero rows in ExemptionRegistry, zero readers, and no roadmap item names the per-example RewriteContext slot its javadoc anticipates. The file’s own precedent (the removed rides-another-rows-key arm) says an arm earns its place through population, so either the arm should be deleted or the anticipated population named by a live plan. Separately, nothing enforces this: ExemptionRegistryTest’s reflective guard finds stray `Map<..., Exemption> fields but not unpopulated taxonomy arms, so the next arm minted ahead of its population is again an audit finding rather than a build failure. Decide the arm’s fate and add the arm-population census (each Exemption permit constructed by at least one registered row, or carrying a stated reason for existing unpopulated).