Function flex_layout 

pub fn flex_layout(
    direction: Direction,
    area: Rect,
    items: &[(Option<u16>, Option<f64>)],
    justify: Justify,
) -> Vec<Rect>

Flexbox-style layout along the main axis.

Each item is (natural_height, explicit_weight):

• natural_height = Some(h) → the component has a content-driven base of h.
• natural_height = None → “no opinion”: treated as a legacy fill participant (base 0, implicit weight 1.0), reproducing the old weighted_split behavior so unconverted/None components are laid out exactly as before.

An explicit weight overrides the implicit one and acts as flex-grow: leftover space (after every measured child gets its natural height) is distributed proportionally to weighted items. Unweighted measured items keep their natural height. When there are no weights and justify is Justify::Stretch, every item grows equally to fill the axis.

Returns positioned, main-axis-sized Rects. The caller still applies cross-axis alignment via apply_align. When justify is Justify::Stretch, the main axis is fully consumed here — do not also call apply_justify with Stretch.

Arithmetic uses i64 internally so that overflow (content taller than the area) and sub-pixel rounding never underflow u16.