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/deferred_generated.rs

@@ -38,6 +38,9 @@ fn deferred_requests_agree_warm_and_cold() {
                         0 => leftover(1).clamp(20, 120).into(),
                         1 => leftover(1).min(rel(0.5)).into(),
                         2 => (leftover(1) + px(30)).min(leftover(2)).into(),
+                        // Both sides an expression, the one shape that
+                        // copies a request's nodes into another's.
+                        3 => leftover(1).min(px(40)).max(leftover(2).min(px(70))).into(),
                         _ => rules[axis].clone(),
                     };
                 }