styled::Separator was a childless Frame with a fixed height, so it
measured to nothing across its list and every caller had to hand it
@sizing=ItemSize::Fixed(SEPARATOR_HEIGHT) to be seen at all, and a
divider in a row could not be drawn that way.

Separator now takes the direction it runs in, thickens that one axis and
leaves the other to its list, with an optional length for a row that
centres its children rather than stretching them. Telling a Frame an
axis takes no measurement needs a prop that can carry None reactively,
so its width, max_width, height and aspect_ratio are Prop<Option<f32>>
with IntoProp lifting a plain value or a signal of one into Some.


Claude-Session: https://claude.ai/code/session_01SUS8o67xj2p5ypsvx1XDqP

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: be3-merge-queue[bot] <332166134+be3-merge-queue[bot]@users.noreply.github.com>