Cover joining two expressions, and print a request one way

Four findings over 05e6ced, which no earlier round reviewed.

`SizeRequest::join` grafts the other side's nodes into this side's arena,
which happens only when both sides are expressions -- nothing in the suite
did that, so the whole thing passed with a `panic!` in that arm. It is
where a missed renumbering would be silent, since an operand copied
without remapping still names a node that exists. A fixture at two window
widths with absolute geometry covers it now, and the deferred corpus puts
an expression on both sides of one arm; both were checked to reach it by
instrumenting again. The path was already right.

`SizeRequest` grew a `Display` because a derived `Debug` of an arena is
not a tree anyone can write out again, and `describe` moved onto it -- but
`Debug` stayed derived, so the `assert_eq!`s in `cases/deferred.rs`, the
only place a request is compared, still printed the arena on failure.
`Debug` forwards to `Display`.

`Nodes::linear` asked nothing of the arena beside it: it is `Operand`'s
question, the way `RequestedLen::linear` is `RequestedLen`'s.

`describe`'s `|r| format!("{r}")` shadowed the `r: &SizeRule` four lines
above it.

Format, workspace clippy under -D warnings with and without
layout-diagnostics, 207 ordinary and 211 diagnostic tests, the cold dump
byte-identical to 05e6ced across all 34,986 boxes, and 400 depth-5 trees
in each of the three deferred corpora in 200.95s.

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

tests/cases/deferred.rs

@@ -132,6 +132,27 @@ fn a_deferred_comparison_can_compare_two_different_weights() {
     assert_corners!(h, b, (200, 0), (300, 100));
 }
 
+#[test]
+fn a_comparison_between_two_comparisons_keeps_both_of_them() {
+    // Joining two expressions is the one path that copies a request's nodes
+    // into another's arena; the floor puts a node under the copied one, so
+    // its operands have to be renumbered as they land.
+    let capped = leftover(1).min(px(40));
+    let floored = leftover(2).min(px(70)).max(px(10));
+    let mut h = Harness::new((90, 100));
+    let both = rect(Color::RED).width(capped.max(floored)).add(&mut h.rsc);
+    let rest = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((both, rest).span(Dir::RIGHT));
+    // Under either cap, so the doubled share is the longer of the two.
+    assert_corners!(h, both, (0, 0), (60, 100));
+    assert_corners!(h, rest, (60, 0), (90, 100));
+    h.resize((300, 100));
+    h.frame();
+    // Over both caps, so the comparison is between 40 and 70.
+    assert_corners!(h, both, (0, 0), (70, 100));
+    assert_corners!(h, rest, (70, 0), (300, 100));
+}
+
 #[test]
 fn a_length_expression_is_resolved_before_wrapping_text() {
     let mut h = Harness::new((300, 500));