Function push_external_property 

pub fn push_external_property(key: PropertyKey, value: PropertyValue) -> bool

Sends a typed (PropertyKey, PropertyValue) write from any thread.

Picked up on the next tick by drain_external_properties; bypasses the legacy Signals mirror entirely so the receiving cell stores the typed variant (I64, F64, Bool, Color, …) directly.

Returns false when the channel has disconnected.