Add topk

[dcherian]

• [ ] add group starts to kth
• [ ] nantopk & topk; which is public?
• [ ] edit CLAUDE.md
• [ ] add numpy to topk definition

This is pretty ugly:

1. Doesn't handle topk and nantopk yet
2. Unlike quantile, topk can work as a tree-reduce algorithm. So we have to hack the k kwarg, passed in finalize_kwargs in many places. This isn't an issue for quantile, because that only works blockwise

[dcherian]

Note to myself: this needs some harmonizing of _DUMMY_AXIS is -2 and the fact that the k-axis is 0