PAPER-FBT08B: Finite Readable Phase Capacity and the Structural Origin of the Vacuum Propagation Bound

This paper studies a possible structural origin of the finite vacuum propagation bound c∗ used in the Lorentzian bridge paper FBT08A. The aim is deliberately limited. We do not derive the numerical value of the physical speed of light, nor do we claim a complete microscopic transport formula for c∗. Instead, we isolate a dual-phase and affine-(S)-sector mechanism inside the FractureBerryTension (FBT) framework that makes the finiteness of the effective vacuum propagation bound geometrically natural. The argument combines three ingredients from the reduced dual-phase sector. The first is a finite positive minimal phase action quantum 0 < hphase < ∞, arising from the quantised action lattice of the dual-phase torus. The second is a finite admissible structural multiplicity Nstr = 24, whose logarithmic form is ln 24. The third is the affine-central normalisation of the residual (S)-sector established in FBT02A, where the (S)-gate is identified as a strict affine central generator whose level is fixed by the normalised Berry flux k =1/2π󰁝Σ2Ωphase. Taken together, these data define a finite readable phase capacity Cphase < ∞ for the observable dual-phase vacuum sector. The main theorem is a conditional finiteness theorem. If the observable Berry readout in the vacuum regime is controlled by this finite phase capacity and the phase-locked vacuum clock sector is nondegenerate, then the induced propagation-rate ratio is bounded above by a finite structural propagation constant κprop < ∞. Here the vacuum clock is not a primitive background time. It is the globally comparable clock readout of the Berry-locked vacuum sector, in the sense of the OMEGA classification of temporal layers. A further finite readout-calibration map then identifies this structural ceiling as the capacity-level precursor of the effective vacuum bound c∗ appearing in FBT08A. Accordingly, the role of the present paper is not to replace FBT08A, but to support one of its central kinematical assumptions. FBT08A studies the geometric consequences of a finite null-propagation bound. FBT08B explains why the existence of such a finite bound is natural from the point of view of the dual-phase readable micro-geometry and the affine-central normalisation of the (S)-sector.