Function assign_value_inplace 

pub fn assign_value_inplace(key: &str, value: impl Into<Stored>)

assigns a Stored value ‘inplace’: replaces the front value in whatever frame it was originally assigned in, without recording an undo entry. This matches Perl’s assignValue(key, value, 'inplace'). Used for MODE changes in enter_horizontal (switches mode without creating a new binding).