Merge Sorted Streams / Top-K

Question

## Problem

Merge k sorted event streams into one ordered stream, then return the top-k by a score.

## Example
```
k streams of timestamped events -> merged ordered; top 10 by value
```

## Constraints
- k can be large

## Approach
Min-heap of stream heads; second heap for top-k. Discuss O(N log k).