LUS gadget: think about supporting more events in script

Sometimes you need to make a standalone gadget, even to handle some logic that only pertains to a single unit type. Think a custom command which requires `CommandFallback`, or maybe some amphibious unit wants `UnitEnteredWater`. I think ideally all `gadget:UnitFrobbed(unitID, foo, bar)` would also be available as `script.Frobbed(foo, bar)`.

The task is to investigate this prompt and come up with a design/implementation.

Start by tracing the gadget event entry points named in the issue, including gadget:UnitFrobbed, CommandFallback, and UnitEnteredWater, and compare them with the script.Frobbed form. Produce a design for exposing more gadget events to standalone scripts, with an implementation plan or implementation that covers the requested event flow.

Written by the indexing model from the issue text.