Drive rules

Pack-root drive: predicate → rule. Runtime auto-drivers: typing, focus, choice_open (not saved).

"drive": {
  "focus": { "use": "happy", "priority": 10, "scale": 1.05 },
  "typing": { "modulate": "#ffffff", "priority": 5 }
}

Usually you need

Field	Effect
`use`	Switch to another expression while the predicate holds
`modulate` / `scale`	Tint / size while active

Field	Type	Effect
`use`	expression id	Switch variant
`modulate`	hex color	Tint
`scale`	number or `[sx,sy]`	Scale
`offset`	number or `[dx,dy]`	Offset
`rotation_degrees`	number	Rotate
`priority`	number	Higher wins when several match
`transition` / `_in` / `_out`	object	Tween (`duration`, `ease`, `curve`, optional `kind`)

Matching rules: modulate/scale multiply; offset/rotation sum.

Layer drive strings only gate visibility — Expressions and layers. Distinct from flag Binds. Characters.