add top-k to MatchMerger

Currently, the MatchMerger collects the matches from the shards and appends all of them to a single output.
This might be confusing to the user since they get `top-k * num_shards` results.

Please implement the top-k into the match merger

Locate the MatchMerger implementation and inspect how it aggregates matches from shards. Implement the requested top-k behavior so the final output is limited to top-k results rather than top-k multiplied by the number of shards, then run the relevant existing tests if available.

Written by the indexing model from the issue text.