Use php in coverage target

[ohmyfelix]

Summary

Replace phpdbg with php in the coverage target so coverage runs follow the org-wide migration in contributte/contributte#73.

Motivation

phpdbg is being removed from Contributte coverage targets. This keeps barelib aligned with the new coverage command.

Changes

• switch both coverage target variants in Makefile from -p phpdbg to -p php

Testing

• make tests
• make coverage (fails locally: no Xdebug/PCOV extension available for php in this environment)
• CI passes