Function begin_pointer 

pub fn begin_pointer<'input, const BORROW: bool>(
    wip: Partial<'input, BORROW>,
) -> Result<(Partial<'input, BORROW>, PointerAction), DessertError>

Prepare a smart pointer for deserialization.

This handles the common logic of:

1. Detecting string pointers (Cow<str>, &str, Arc<str>, Box<str>, Rc<str>) which should be handled as scalars
2. Calling begin_smart_ptr() for other pointer types
3. Detecting whether it’s a slice builder or sized pointee

Returns the prepared Partial and an action indicating what the caller should do next.

Usage

match begin_pointer(wip)? {
    (wip, PointerAction::HandleAsScalar) => {
        // Handle as scalar/string - set_string_value handles all str pointers
        deserialize_scalar(wip)
    }
    (wip, PointerAction::SliceBuilder) => {
        // Arc<[T]>, Box<[T]>, etc - deserialize list items
        let wip = deserialize_list(wip)?;
        wip.end()
    }
    (wip, PointerAction::SizedPointee) => {
        // Arc<T>, Box<T> - deserialize inner
        let wip = deserialize_into(wip)?;
        wip.end()
    }
}