What Clocks Already Exclude: A Coupling Bound for the Fifth Coordinate from Optical-Clock Sensitivity

If the fifth coordinate couples to ordinary matter, precision clocks should notice. This paper converts the sensitivity of modern optical-clock comparisons into a bound on that coupling. Assuming an advance Dchi of the phase shifts a clock's fractional frequency by g_chi Dchi, and taking the unexplained-shift sensitivity as 1e-18, the coupling is bounded by g_chi <= 1.592e-19 per radian for a full turn and g_chi <= 1.499e-17 per radian for a single 0.0667 rad step. Conversely, a coupling of 1e-12 per radian would shift clocks by 6.283e-12 over a turn, more than six million times the sensitivity floor, and is therefore excluded by existing clock data under the stated model. The bound does not exclude transit; it excludes transit that drags clock rates with it at any coupling above the 1e-19-per-radian scale. Computed from stated assumptions and a stated sensitivity figure; not a new measurement. SIMULATED, NOT MEASURED.