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>

tests/prelude_bounds.rs

@@ -0,0 +1,13 @@
+//! The prelude must not shadow names from the standard prelude. A widget
+//! called `Sized` meant every `?Sized` bound in a crate using iris resolved
+//! to the widget and failed to compile, which is an error in someone else's
+//! code that nothing here would have noticed.
+
+use iris::prelude::*;
+
+fn takes_unsized<T: ?Sized>(_: &T) {}
+
+#[test]
+fn the_prelude_leaves_sized_alone() {
+    takes_unsized("a `?Sized` bound resolves to the marker trait");
+}