Method execute_custom_shard_function

Execute a custom shard function using the jssg engine (QuickJS). The user's script exports a default function that receives a typed `ShardInput` and returns `ShardResult[]`. The engine handles all file I/O and serialization.

(
        &self,
        shard_config: &butterflow_models::step::UseShard,
        func: &butterflow_models::step::CustomShardFunction,
        eligible_files: Option<&[String]>,
        previous_shar