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/position/set_size.rs

@@ -0,0 +1,34 @@
+use crate::prelude::*;
+
+pub struct SetSize {
+    pub inner: StrongWidget,
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl SetSize {
+    fn apply_to_outer(&self, ctx: &mut SizeCtx) {
+        if let Some(x) = self.x {
+            ctx.outer.x.select_len(x.apply_rest());
+        }
+        if let Some(y) = self.y {
+            ctx.outer.y.select_len(y.apply_rest());
+        }
+    }
+}
+
+impl Widget for SetSize {
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.widget(&self.inner);
+    }
+
+    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+        self.apply_to_outer(ctx);
+        self.x.unwrap_or_else(|| ctx.width(&self.inner))
+    }
+
+    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
+        self.apply_to_outer(ctx);
+        self.y.unwrap_or_else(|| ctx.height(&self.inner))
+    }
+}