fix: expose `Array.back`, `back!`, and `back?`

This PR marks `Array.back`, `Array.back!`, and `Array.back?` as `@[expose]`, so that in a downstream module their bodies are available for definitional reduction. Previously `decide` (and even `decide +kernel`) could not evaluate goals such as `#[1, 2, 3].back? = some 3` from another module, even though the `getElem?`/`size` accessors these functions are defined in terms of, and the neighbouring `Array.singleton`, are already exposed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>