Routes events to workers by hashing stream_id.

Same stream_id always hashes to the same partition for a fixed number of partitions. :erlang.phash2/2 distributes uniformly across the result range, which is what the per-stream-ordering guarantee in §3.2 requires.

This is plain modular hashing, not ring-based consistent hashing. The number of partitions is fixed at projection start (parallelism: N); when it changes, the projection is restarted from scratch (or, more typically, a new version is started side-by-side and cut over). Ring-style minimal remapping has no use case here.