Cosmological Population and Transport Extension of the Neutrino Standing-Carrier Space

Neutrino carrier ARC and HNL prediction 12/18 This paper applies the PCR‑5C target-extension calculus to the cosmological population and transport behavior of the neutrino standing-carrier space. The preceding PCR papers establish the neutrino oscillation carrier quotient, its source-first realization space, simultaneous physical survivor residue, compatible role-occupancy structure, and target extensions involving direct kinematic mass, lepton-number violation, heavy states, and active-sector closure. PCR‑5D4 extends this structure to cosmological population, propagation, stress-energy, and transport. The analysis constructs a source-faithful cosmological extension containing the relevant neutrino population slots, mass records, standard thermal distributions, weak-decoupling initial data, free-streaming evolution, cosmological background attachment, finite transport hierarchies, response and provenance objects, and null terminal objects for off-seed sectors. A central result is the Source-Faithful Kinetic-Saturation and Stress-Energy Descent theorem. It establishes closure of the retained population support under production, collision, decay, injection, attachment, and constituent relations, together with the induced stress-energy descent. On the transparent branch, the kinetic support coincides with the positive population support, off-support contributions vanish, and the resulting stress-energy is formed without omission or double counting. Opaque branches are retained with explicit witnesses of non-descent rather than being silently treated as transparent. The paper also constructs a seeded transparent cosmological section for each source-qualified carrier representative. The seed-active extension includes three population slots, their mass records, Fermi–Dirac distributions, weak-decoupling data, free-streaming operations, cosmological background attachment, a certified finite hierarchy, and the required response and provenance structure. This provides an explicit relative cosmological realization while preserving the underlying oscillation carrier and its target-facing trace. The resulting source-to-cosmology construction is compatible with the retained oscillation structure and distinguishes carrier identity, population realization, transport behavior, stress-energy response, and cosmological observation. The extension therefore tests how the standing-carrier space can support a populated cosmological sector without conflating a population model with a new carrier identity. The paper establishes a source-faithful cosmological population and transport extension, together with transparent-section realizability and stress-energy descent on the declared branch. It does not by itself supply a complete cosmological likelihood, a uniquely selected carrier, a fully resolved cosmological mass-sum inference, or a complete observational endpoint. Those require the subsequent cosmological response and likelihood target. The conclusion is deliberately bounded. The result is relative to the fixed PCR‑5B standing-carrier space, PCR‑5C extension calculus, prior physical survivor structure, declared cosmological source ontology, and specified population and transport target. It does not establish a unique cosmological history, a unique neutrino mass mechanism, or a complete cosmological parameter determination.