Add microbenchmarks for `PointerUnion`'s `isa` and `isNull` operations across union sizes of 2, 4, and 8 types. This it to establish baseline performance numbers before making changes to the implementation. I plan to refactor the implementation a bit and add support for more variants using sparse encoding. Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>