perf: Avoid freshening the same predicate twice per selection

[xmakro]

Selection freshens the obligation predicate once when pushing it onto the stack, then freshens the same predicate again in candidate_from_obligation to build the cache key. Both happen before the cache is probed, so the second fold is paid even on hits.

When the predicate contains no type or const inference variables the two results are identical: the freshener's only state-dependent paths are its inference variable arms, and its region handling is stateless erasure. The gate is therefore on non-region inference variables rather than on inference variables generally, since region variables cannot reach the stateful paths and are pervasive in selection obligations.

The freshened predicate is a cache key, so a debug assertion recomputes the fold and compares. A future change to the freshener that breaks the equivalence fails loudly under debug assertions instead of silently corrupting the selection cache.