Route a Query to the Correct Shard (Practical)

Question

## Problem

Given a sharding scheme (key ranges or hash), implement routing of a query/key to the right shard, and a scatter-gather for cross-shard queries.

## Example
```
key -> shard via range/hash; cross-shard aggregate via gather
```

## Constraints
- Handle resharding boundaries

## What PlanetScale looks for
Vitess-grounded: shard routing and scatter-gather are core to PlanetScale's distributed MySQL.