Q Quantum Theory II-D: Electrodynamics - Phenomenology and Experimental Tests

Q Quantum Theory II-A develops the physical finite-coherent electrodynamic sector (QQED), while Papers II-B and II-C provide the technical and representation-level closures in which the charged Q is a complete quantum object and electromagnetic interaction is generated by one gauge-complete finite-response mother. The present paper isolates the phenomenological content of that construction and turns it into a hierarchy of parameter determinations, parameter-free hold-out predictions, and falsification tests. The central spectral prediction of the canonical same-Q completion is an affine internal response mass-squared ladder,\[(E_n^{(0)})^2=m^2+n\gamma_C,\]where $n\in\mathbb N_0$ is the response level, $m$ is the ground-state mass, $\gamma_C>0$ is the mass-squared spacing and $E_n^{(0)}$ is the bare rest energy in units $\hbar=c=1$. The corresponding observable is not a resonance maximum but the centroid of the complete Feshbach seed spectral measure. The exact centroid sum rule removes arbitrary dispersive self-energy shifts. Finite-window, finite-sample, repeated-look and detector-response extensions are derived without assuming asymptotic completeness. Relative bright one-photon branching fractions reduce to a two-parameter shape law in $(x,\mu)$ with $x=R\sqrt{\gamma_C}$ and $\mu=m^2/\gamma_C$, where $R$ is the interaction-response width, whereas the overall bright lifetime scale is controlled by a common transverse coefficient. Once the ground mass is known externally, only two newly resolved response excitations are required for the first parameter-redundant spectral test. Additional response levels, branching coordinates, cascade observables and lifetime ratios overconstrain the same parameter set. A selection-safe discovery-confirmation protocol, calibrated detector duals, and a cross-scale bridge from low-energy determination of $R$ to high-resolution finite-response tests are given. A dated experimental-status audit separates directly calculable Pauli hold-outs from atomic, antimatter, strong-field and collider correspondence targets for which a QQED observable must still be derived before numerical parameter bounds are legitimate. The scope of falsification is stated explicitly: failure of one canonical QQED phenomenological completion is not logically identical to falsification of the Paper-I Q ontology, and experimental failure of a physical realization of the finite-coherent ultraviolet mechanism is distinct from a mathematical counterexample to a conditional ultraviolet-convergence theorem.