Design Constraints Conditional on the Spin Bath: An engineering layer for crystalline mesoporous forsterite after the locked Host-evaluation gates

We specify a conditional engineering layer for crystalline mesoporous Mg₂SiO₄ (Gao-type; ordered pores ~2.58 nm) that may be used only after the locked Host-evaluation module of the parent design paper has returned a measured spin-bath profile. The parent paper supplies the characterization protocol, the numerical keep-versus-scaffold gates, and the default encoding stance. The present work supplies the protection, encoding, and architectural choices that remain consistent with those gates. We do not report a measured paramagnetic density, nuclear spectral density, two-level-system (TLS) spectrum, or coherence time. The bounding quantities are stated as design targets and an expected bath hierarchy. Regime C (hybrid / external encoding on ranks 1–4) remains the current architecture. Regimes A and B are specified only as conditional designs, each gated by falsifiers F1–F3. A design rule follows: natural-abundance ²⁵Mg cannot be treated simultaneously as the dominant nuclear bath to be removed and as the native I = 5/2 (d = 6) resource. Optical cat-code or cavity encodings are not assigned to the 2.58 nm pore interior. This is a companion manuscript to “Silicate Pathway to Durable Quantum Hosts” (design paper v1.1, 12 August 2026).