Function batch_verifier 

fn batch_verifier(
    vk: &'static ItemVerifyingKey,
) -> Fallback<Batch<Verifier, Item>, OrchardFallback>

Builds a global Halo2 verifier that validates every item against vk.

The returned service batches contemporaneous proof verifications and, if a batch fails, falls back to verifying each item individually. The batch and its fallback share the single vk passed here, so an item built by this verifier is always checked against exactly one era’s key. Callers select the correct era’s key by which VERIFYING_KEY_* they pass (see the two statics below); there is no runtime key resolution.