Charge-Uncompensated Isocurvature in an Interacting Dark Sector: Primordial Regularity and Late Transverse Conversion

This paper studies how a standard baryon–cold-dark-matter compensated isocurvature perturbation (CIP) can remain uncompensated with respect to an interaction charge that distinguishes CDM from baryons in a pressureless Type-2 interacting dark sector. On a compensated initial slice it derives an exact scalar source, K chi_NN = 3 Omega_b beta_eff Delta, while the source vanishes in the uncoupled limit. The early-time transverse sector is classified by primordial regularity: radiation-era boundedness removes the singular intrinsic branch, while the ordinary CIP supplies an independent regular external source whose conversion into the endpoint-regular transverse coordinate begins at O(a^4), earlier than the ordinary adiabatic Type-2 rider at O(a^6). The analysis then constructs a source-correct finite-k system, reconstructs gauge-clean matter and composition variables, and embeds the same fixed action in a photon–baryon–neutrino hierarchy. The exact conformal-Newtonian Type-2 perturbation block is implemented in a pinned CLASS v3.4.0 tree, with byte-identical selected outputs when the Type-2 sector is disabled and production propagation of adiabatic and direct-CIP columns for gamma = 0,1,2,3,4. The direct-CIP column is assigned an independent primordial amplitude, tilt and curvature correlation, and its covariance implementation is numerically verified. The paper does not infer a coupling from Planck data and does not compute the full second-order action-jet observable.