Hold the output as the box every chain bottoms out in

A position was composed up the slot chain to a normalized region and then
multiplied by the output's size, so the window was the one box in the
system that was not a box. Seeding the chain with a root slot holding it
in pixels makes composing through it leave everything below in pixels,
which is what the multiplication was doing.

`within` already does the arithmetic: a child at `rel` 1 inside a span of
`px` 0 to `px` 1920 composes to `px` 1920 and `rel` 0, so the trailing
`to_px` becomes the identity rather than a step. The shader walks the
same chain and needs no change for the same reason.

This is the shape the resize machinery wants before it can be deleted: a
resize becomes one slot written, which `try_reuse` and `redraws_under`
already carry. Nothing is removed yet.

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

tests/layout.rs

@@ -180,5 +180,9 @@ fn only_a_container_that_places_its_children_lengthens_the_chain() {
     h.set_root((bar, buried).span(Dir::RIGHT));
 
     let slot = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(slot), 1, "one span above the leaf");
+    assert_eq!(
+        h.render.moves.depth(slot),
+        2,
+        "the span above the leaf, and the root the window is held in"
+    );
 }