Function plan_vector_split 

pub fn plan_vector_split(
    source_vshard: u32,
    new_vshard: u32,
    target_node: u64,
    document_ids: &[String],
) -> SplitPlan

Plan a vector-aware shard split.

Splits documents by partition key: documents whose key hashes to the lower half stay on the source shard, upper half moves to the new shard. Each resulting shard builds its own HNSW index independently.

Cross-shard k-NN: the Control Plane dispatches VectorSearch to both shards via scatter-gather, collects top-k from each, and merges by distance on the Control Plane.