Batch.size

An immutable batch plan. Construction does no tmux I/O and never touches a shared Java object: each add returns a new value holding one more argument vector. Each run / length builds a fresh Java Batch from create and dispatches it through Execution, so it is F-suspended, cancellation-safe, and admission-bounded like every other Cats operation — cancelling one run never disturbs a concurrent one, since neither shares Java state.