Custom VFX for item def's

As the discussed example

the ability to add a passive custom vfs to items that would appear at the actors model (eg smoke clouds for IC engines)

No files, tests, or entry points are named. Start by locating how item definitions and actor-model effects are represented, then clarify the expected passive custom VFX behavior and lifecycle; done means an item can specify an effect that appears on its actor model, using the smoke-cloud example as the acceptance case.

Written by the indexing model from the issue text.