Ask a value a question through a reference

A method taking `self` can only be called on a value, so anywhere the caller
holds a reference it has to dereference to ask -- which costs the caller
whether or not the type is `Copy` (Bryan, 2026-09-20, correcting the opposite
change made in 7502176).

So every method that answers a question about a value takes `&self`:
`Holds`'s four, `AxisHolds` and `LayoutHolds`'s three each, `LayoutLen`'s
`is_px`, `is_only_leftover`, `declared` and `fills`, and `Size::within_box`.
The two callers passing `LayoutLen::declared` as a function value say the
closure instead.

Builders that return a changed copy, and methods on a handle that is meant to
be given up, still take `self`.

core/src/orientation/len.rs

@@ -148,20 +148,20 @@ impl LayoutLen {
     /// Only pixels: the same number of them whatever box it lands in, and
     /// whatever anyone else in the row asks for. A length that is any part
     /// of a box or of what is left over is not one.
-    pub fn is_px(self) -> bool {
+    pub fn is_px(&self) -> bool {
         self.rel == Rel::ZERO && self.leftover == Weight::ZERO
     }
 
     /// Nothing but a claim on what is left over, so there is no length here
     /// at all where nothing is.
-    pub fn is_only_leftover(self) -> bool {
+    pub fn is_only_leftover(&self) -> bool {
         self.leftover > Weight::ZERO && self.without_leftover() == Len::ZERO
     }
 
     /// This as a length of a box, where it is one. `leftover` is not: a
     /// share of what is left over is a length only to whoever divides one,
     /// so it passes up in the reported size instead and is resolved there.
-    pub fn declared(self) -> Option<Len> {
+    pub fn declared(&self) -> Option<Len> {
         (self.leftover == Weight::ZERO).then(|| self.without_leftover())
     }