The de Sitter-Unruh Modified-Inertia Theory: A One-Parameter Account of the MOND Acceleration Scale

I present the de Sitter-Unruh modified-inertia (MI) theory as a self-contained, ONE-PARAMETER physical account of the MOND acceleration scale a0 -- explicitly NOT a theory of everything. The theory rests on two physical inputs. First, an accelerated body immersed in a de Sitter universe sees a thermal bath at the Deser-Levin temperature T (a) = (hbar/2pi kB c) sqrt (a² + (cHLambda) ²), which never falls below a universal floor T0 = hbar cHLambda/ (2pi kB c) = 2. 20e-30 K set by the dark-energy horizon alone. Second, a relational/Machian principle: a body's force-responsive (dynamical) inertia is the EXCESS of its bath response above that universal floor -- a body unaccelerated relative to the cosmic vacuum carries no dynamical inertia. From these two inputs, four structures follow as consequences rather than fits: (i) the SCALE a0 = cHLambda/Z ~ sqrt (Lambda) ; (ii) the INTERPOLATION FORM mufw (x) = (sqrt (1+4x²) -1) /2x, with the strikingly clean inverse mu/ (1-mu²) = a/a0, realized as the minimum of a convex, ghost-free potential; (iii) the MEMORY KERNEL theta (0) = sqrt2 (amplitude-branch selection) with a Lorentzian shape forced by the de Sitter Wightman function; and (iv) the MOND SIGN itself -- the relational principle selects inertia-weakening (MOND) and rejects the passive-bath, inertia-raising (anti-MOND) reading that the influence functional otherwise gives. What remains is a single free number Z = 2 sqrt (8pi/3), provably free (the kappa-closure result), entering exactly as Newton's G enters general relativity. The honesty is explicit and two-sided throughout: the sign is CONDITIONAL on the Machian premise (the honest crux) ; Z and hence a0's numerical value are NOT derived; and the Standard-Model mass sector stays WALLED (the Koide relation is beyond symmetry, the Yukawa sector kernel-free) -- this is a one-parameter theory of one infrared scale, not a theory of everything. The theory is consistent with, and assembles, the author's previously published effective-field-theory results (DOIs 10. 5281/zenodo. 20935948, 20938891, 20947913, 20963226) and with the author's 2026 retraction of all earlier theory-of-everything / Standard-Model overclaims. Predictions: an sTX Lorentz-violation dipole (~1. 5x the tightest current bound), an a0 (z) 'hostage' tied to the dark-energy equation of state, a modified-gravity-impossible dwarf orbital-history velocity-dispersion signature (+12-14%), and a definite universal floor temperature T0 = 2. 20e-30 K. All results are reproducible from the public repository.