Decouple pathing push mass and physics mass

Apparently units bias their mutual push behaviour by their relative masses. I would like to be able to tweak push behaviour in this way, but cannot as I already use the mass tag to define how resistant a unit is to impulse.

No files or tests are named. Start by tracing where unit mass affects pathing push behaviour and where it affects physics impulse resistance, then identify the shared mass handling. Done means push behaviour can use a separate mass value without changing physics mass semantics.

Written by the indexing model from the issue text.