Type the callable case of Option::ensure()

[GrahamCampbell]

This stacks on #85 and types the callable case of ensure(), which #84 left as a bare callable in the union, so a closure argument bound the template to the closure type itself rather than to what it returns; ensure(fn (): string => 'x') now infers Option<string> instead of an option of the closure. Because the annotation now declares callable():(S|Option<S>), static analysers will begin flagging two patterns that were previously accepted: callables that require arguments, and literal function-name strings passed as the value. Both of these already fail at runtime, because ensure() invokes callables with zero arguments and treats a string as a plain value only if it is not callable, so the new errors point at pre-existing latent bugs rather than breaking valid code, though consumer baselines with entries mentioning ensure() may need regenerating, and the release notes should mention the change. Callables whose arguments are all optional remain accepted. This is an annotation-only change with no runtime impact, so no tests are added, and the library's own PHPStan baseline is untouched.