Quantum Structural Theory of Harmony (QSTH M.1B-SR1) — Spectral-Response Bridge for a Dirac Carrier at an Interface

QSTH M.1B-SR1 is a separate companion study following the B7 integration audit of QSTH M.1B. It tests a more informative alternative to a simple sector count: whether the thermal entropy of specified interface carriers can be related to an independently defined physical response derived from the same microscopic spectrum. The model consists of free complex Dirac fermions localized on a two-dimensional interface. For the massless planar limit, at zero chemical potential and with no surface mass gap, the study derives S_D = R_D k_B² T χ_Qwith R_D = 9 ζ(3)/(4 ln 2) = 3.901953449… where S_D is the thermal entropy of the Dirac carriers and χ_Q is their independently defined charge susceptibility. The coefficient is not fitted: it follows from the ratio of two Fermi integrals evaluated over the same prespecified spectrum. Entropy and susceptibility therefore have independent operational definitions while sharing the same Hamiltonian, ensemble and charge normalization. The result is intentionally model-specific. The audit shows that the simple coefficient is not universal on a finite sphere, at nonzero chemical potential, after opening a mass gap, or after changing the statistical preparation. An independent neutral sector may add entropy without contributing to the charge response, while a strictly fixed-charge ensemble may retain positive entropy with vanishing susceptibility. The study therefore establishes a conditional spectral-response bridge, not a universal horizon law. It does not identify the Dirac carrier with the physical QSTH horizon carrier, does not identify its thermal entropy with the full horizon S_eff, and does not derive M-independence, E10 or E11+. The accompanying reconstructed reproducibility package checks the localized mode, planar integrals, finite-sphere sums, charge response, direct Fock-space traces, additional species, neutral factors, fixed-charge ensembles, gaps and spectral cutoffs.. The purpose of SR1 is to demonstrate how a future physical model may connect microscopic spectrum → independent response → entropy without defining one side through the other. The package was reconstructed from the final SR1 publication and is explicitly distinguished from the original 28 September 2026 audit archive.