Active Transport on the Prime Gas: Flat-Band Condensation, the Rabi Phase Transition, and the Arithmetic Qubit

We construct the Hermitian active transport operator H(α) = D_sym/λ_P + iα[D_sym, P_τ]/λ_P on the coprime residues modulo a primorial m, where D_sym is the palindromic distance matrix, P_τ is the multiplicative inversion permutation, and α is a real shear parameter. Full eigendecomposition at four primorial levels (matrices up to 5760 × 5760) reveals seven new spectral phenomena: (1) flat-band condensation with 99.7% of eigenvalues collapsing to zero; (2) Perron gap universality converging to 0.8146; (3) a sharp phase transition at critical shear α_c ≈ √(135/88) with transition width 10⁻⁷ and derivative -4,989,133; (4) Sylow torsion defects that persist despite primorial expansion; (5) selective screening of prime-periodic perturbations; (6) CRT block architecture providing exact scalar protection within 30 blocks of dimension 16; and (7) exact σ-parity conservation, which reveals that the system is an arithmetic qubit with computational basis states given by τ-parity within each σ-sector, protected by four layers of number-theoretic error resistance. The phase transition at α_c is a Rabi crossing — the 50/50 coherent superposition point between additive standing waves and multiplicative chiral currents. Companion paper to three prior preprints on palindromic distance matrices, primorial thermodynamics, and spectral isotropy.