Rename the Sized widget to SetSize

It shadowed the marker trait, so a `?Sized` bound in any crate that imports
the prelude failed to resolve -- a compile error in someone else's code that
nothing here would have caught. Three files inside iris already imported
`std::marker::Sized` to get out from under it; they no longer need to.

`SetSize` rather than `FixedSize` because the size it sets need not be fixed:
`width(rest(2))` and `width(rel(0.5))` build the same widget. It pairs with
the `MaxSize` beside it -- one sets a length, the other caps it.

`tests/prelude_bounds.rs` is a compile-level guard: it fails to build if the
prelude shadows `Sized` again. The pad tab of the tabs example, which is what
uses `sized` and the flexible widths, renders pixel-identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/ptr.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::{Sized, Unsize};
+use std::marker::Unsize;
 
 pub struct WidgetPtr {
     pub inner: Option<StrongWidget>,