Phenomenology of EDE-photon coupling I: constant photon-sector deviation

We investigate a phenomenological extension of the photon sector motivated by an early-time interaction between a scalar field component and radiation. The model is described by a constant parameter $\epsilon$ which measures the departure of the photon energy density and CMB temperature-redshift relation from their standard adiabatic evolution. The standard photon sector is recovered when $\epsilon=0$. We implement the modified photon scaling in CLASS and verify that the numerical background evolution agrees with the analytic constant-$\epsilon$ prediction. We then study the diagnostic response of the recombination history, visibility function and CMB temperature spectrum. These diagnostics show that small values of $\epsilon$ can shift the recombination history and modify the acoustic peak structure of the CMB temperature anisotropy spectrum. These CMB outputs are used only as consistency and response diagnostics not as full CMB likelihood constraints. As a preliminary statistical application, we combine the modified CLASS implementation with MontePython and constrain the model using Pantheon+SH0ES supernova data together with BAO distance measurements. The late-time analysis gives $\epsilon=0.0230\pm0.0065$ and improves the best-fit likelihood relative to the fixed $\epsilon=0$ baseline with $\Delta\chi^2_{\rm eff}=-10.518$ and $\Delta{\rm AIC}=-8.518$. These results indicate that the constant-$\epsilon$ extension is favoured by this preliminary late-time data combination according to the AIC criterion.