Enhance EPC equivalence fit output and preserve miPowerFit compatibility

[psunthud]

This pull request updates the former miPowerFit() functionality to correct the EPC equivalence testing procedure by introducing an explicit underpowered decision category. The method is also revised to evaluate residual correlations, such that residual covariances are scaled by residual variances rather than by total indicator or factor variances.

A new summary() method is implemented for the equivalence-testing output, providing concise summaries of each fixed parameter’s classification as well as the resulting global decision.

The function has been renamed to epcEquivFit() to better reflect its purpose, while full backward compatibility with miPowerFit() is preserved.

In addition, a new function, epcEquivCheck(), is introduced to assess whether trivial misspecification is difficult to establish for a given model under the EPC equivalence testing framework.

Clipboard utilities have been updated to accommodate these changes, and all relevant documentation has been revised to reflect the transition from miPowerFit() to epcEquivFit().

I ran devtools::check() on Windows (R 4.5.0); checks pass with 0 errors.