v0.0.0-9938-g8a615d9d5

`const`-ify APIs of `UnionFind` that would be expected to be `const`

The only reason they aren't is because we do path halving for `FindRoot`, but that's an implementation detail and better captured by marking the `nodes_` as `mutable`.

Also tightening the control over values in `Node`.

PiperOrigin-RevId: 911579523