PassengerProcessTask

`construct.passenger.process`: run one step of the post-process chain on a mesh or tet item, as Mesh ▸ and Tet ▸ do (construct-mesh-tet-items §MTO-1, §MTM-4).

In the namespace construct::opal. Declared in src/libs/libconstruct/construct/opal/PassengerProcessTask.hpp.

Detailed description

The same PassengerOperation as the menu: the step runs on the task thread, the new file goes next to the old one, and one undo step sets the new `sourcePath` and records the step. A step that needs a field fails with the chain's own message and changes nothing.