Complement-Sector Feedback and Observable Structure in Q5 Transport

This Theorem establishes the complete observable classification of complement-sector feedback. The complement block of admissible form H_cc = aI + bJ (J² = I) produces a Schur correction ΔH = H_rc·H_cc⁻¹·H_cr that decomposes into: ΔH = (a/(a²−b²))·Δ₀ − (b/(a²−b²))·Δ_J where Δ₀ = H_rc·H_cr and Δ_J = H_rc·J·H_cr. Five results are established. (I) Scalar invariants protected: the Schur correction lands in span{I, σ_z, σ_x} on the transverse sector and is annihilated by the extraction functional L, μ is invariant under complement-sector feedback. (II) Embedded persistence globally protected: the correction lives in the remainder sector Ω_rem paired under the mirror involution J; mirror-orbit cancellation (T38) gives λ_eff(θ) = 1 independently of local coefficient structure. (III) Propagation orbit structure preserved: feedback deforms transport coefficients but cannot generate new directions in O_q(s), orbit dimension is unchanged. (IV) Spectral deformation real but symmetric: the helicity-twisted component Δ_J is governed by the same spectral discriminant ω = √(p² − 4a²) as the T30 dressed channel, identifying Δ_J as the operator-level origin of the T30 spectral deformation. This deformation is annihilated before scalar readout by parts (I) and (II), resolving the apparent tension between the nontrivial T30 deformation and the clean T29 extraction. (V) Sole observable manifestation is spectral redistribution across fixed propagation channels, including spectral line splitting, linewidth asymmetry, and beat modulation at frequency p − ω. Two independent protection mechanisms: local null-class annihilation (μ) and global mirror-orbit cancellation (λ_eff) operate independently. Status: Null-class protection of μ solid in minimal model, ambient T12/T16 derivation of Schur landing remains open. λ_eff = 1 solid, T38 mirror-orbit cancellation operates independently of local structure. Orbit preservation solid via T34–T35. T30 spectral deformation, real and symmetric solid. Spectral redistribution as sole observable structural — not yet derived from full ambient architecture. Two independent protection mechanisms solid. Dependencies: T26, T29, T30, T34, T35, T37, T38.