Phase-Plane Reduction and Scalar Phase Defect

This Theorem extracts the phase-bearing 2-plane from the T23–T25 ambient structure and identifies the scalar phase defect Δφ(τ) with the rotational coefficient of the induced collapsed operator on that plane. Two supporting lemmas are established. Lemma 26.A performs the branch-axis decomposition: the T22 triplet T₃ splits canonically into the branch-selected zero-weight axis span{u₃} and the transverse phase plane P⊥ = span{u₂, u₄}. Lemma 26.B establishes the readout kernel: every Hermitian operator on P⊥ decomposes uniquely in the real basis {I, σ_z, σ_x, R}, and the U(1)-holonomy readout ρ_phase annihilates I, σ_z, and σ_x, retaining only the coefficient μ of the rotation generator R. The main theorem establishes that the collapsed branch object C_A induces an operator Q⊥ on P⊥, the holonomy readout factors through P⊥, and (after fixing the T15 normalization convention) Δφ(τ) = μ exactly. The coefficients α, β, γ drop out of the readout regardless of their values. Status: Parts (1)–(3) solid by direct structural argument. Part (4) inherits the conditionality of T20; the identification of Δφ(τ) as the scalar shadow of the collapsed kernel is a load-bearing assumption carried forward from T20. Dependencies: T15, T18, T19, T20, T23, T24, T25.