Compile multiple programs into an ensemble program.

Each child program is called independently. size: nil and size: 0 both execute every child, matching DSPy's public subset-selection behavior. A positive :size selects that many children. A failed child contributes an {:error, reason} entry to the ensemble outputs instead of crashing the whole ensemble. When a :reduce_fn is supplied, it receives only successful predictions; reducer exceptions or invalid reducer returns become structured {:error, reason} results. When :size selects a random subset, :seed drives a pure per-input RNG stream, so the same saved ensemble and inputs replay the same selection without depending on process-global random state.