Apply WordPress core library overlays before provider activation

[chubes4]

Runtime overlays for WordPress core libraries were mounted but not applied early enough for plugin activation.

Observed progression:

• PHP AI Client overlay was declared in the generated recipe.
• Plugin activation still loaded the older core copy and failed on missing ProviderMetadata::getDescription().
• Moving overlay application later initially hit runtime mu-plugin mount ordering.

Acceptance criteria:

• Core library overlays such as PHP AI Client are applied to the WordPress runtime before provider plugin activation.
• Overlay application runs after the runtime mu-plugin directory is available.
• Recipe diagnostics clearly show whether an overlay was applied.
• Tests cover core overlay copy/application ordering.