Build a small `shard_picker(user_id, num_shards)` using simple consistent hashing: ``` shard_index = sum(ord(c) for c in str(user_id)) % num_shards ``` Returns the integer shard index (0-indexed). This is the simplest possible content-addressing scheme — production uses MD5/SHA1/Jump-Consistent-Hash but the SHAPE of the decision is identical. Test for 3 known IDs against 4 shards.