Internal pseudospin, lepton-number superselection, and neutrino-antineutrino coherence in massive neutral-fermion one-particle states

At fixed three-momentum, massive Dirac neutrino one-particle states span a four-dimensional space of particle--antiparticle identity and helicity. We show that helicity flip, charge conjugation, and their product close an internal $SU(2)$ pseudospin subalgebra within $SU(4)$, distinct from the Wigner little group. Its helicity generator is the lepton-number-weighted spin rotation U₁=2LJ₂. The lepton-number L and helicity label Hₕ grade the sixteen generators τ_μ⊗σ_ν, organizing the Δ L=2 sector. This grading resolves a common ambiguity: helicity-preserving directions τ1,2⊗σ0,3 carry pseudo-Dirac propagation, with coefficient κpD=Δ m²/(4E) and active--sterile conversion, whereas helicity-flipping directions τ1,2⊗σ1,2 carry Majorana mass insertions, neutrinoless-double-beta effective mass factors, and active--active neutrino--antineutrino coherence. We derive the one-particle Majorana projection from the field-level condition, clarify the phase convention, and separate the Lagrangian mass insertion from the induced propagation Hamiltonian. The construction classifies the kinematical channels activated by Majorana masses, pseudo-Dirac splittings, medium-induced spin coherence, or other Δ L=2 dynamics; it does not assume that neutrinos are Majorana particles.