A D-Brane-Induced Quaternionic and PT-Symmetric Spacetime with Falsifiable Signatures

We give a self–contained derivation that upgrades the Dirac–Born–Infeld+Chern–Simons action of a single Type–IIB D3–brane to a four–dimensional, quaternionic and PT –symmetric spacetime model with only two free parameters. A long–wavelength NS–NS two–form induces exactly two linear, SU(2)-valued deformations of the open–string metric, ϵ(t) e1Tμν and ϵ(r) e2Rμν, where ϵ(t) = ϵ0 cosωt and ϵ(r) = ϵ1/r play the role of geometric activators. With the minimal prescription P : xi 7→ −xi, T : t 7→ −t, i 7→ −i the full Dirac operator becomes pseudo–Hermitian and the metric remains PT –invariant. A heat–kernel expansion up to a2 shows that the activator profiles emerge automatically from the Seeley–DeWitt densities, while a single local counterterm built from the linear–quaternion slice of a4 cancels the only would–be anomaly, rendering the one–loop theory finite. The resulting spectral action predicts a narrow phenomenological window, |ϵ0| ≲ 10−7, |ϵ1| ≲ 101, already constrained by PLANCK 2018 CMB data, low–surface–brightness rotation curves, and present atom–interferometer limits. Forthcoming measurements with MAGIS–100, ELGAR and the Einstein Telescope can tighten these bounds by one–to–two orders of magnitude, providing a decisive test of the framework. Conceptually, the work closes the loop DBI(10D) → quaternionic (4D) → spectral dynamics → laboratory/astrophysical observables, and offers a minimal template for exploring higher–dimensional quantum structures in gravity with falsifiable predictions.