Function validate_batch 

pub fn validate_batch<P: RateLimitPolicy>(
    checks: &[Check<'_, P>],
    maximum: usize,
) -> Result<(), BatchError>

Validates backend-independent structural requirements for an atomic batch.

Duplicate counters are reported in caller order: duplicate_index is the earliest repeated input, and first_index is that counter’s first input.

Errors

Returns BatchError::BatchTooLarge before inspecting keys when the batch exceeds maximum. Otherwise returns BatchError::DuplicateKey for the first repeated logical counter.