Function prefill_batched_chunk_zero_admitted 

pub fn prefill_batched_chunk_zero_admitted() -> bool

2026-09-28: Whether a batched prefill wave may carry chunk-zero streams: the first-chunk lever (prefill_batched_first_chunk_enabled) or varlen batching (prefill_varlen_enabled). The single rule for the model engine’s admission (kernel_batched_eligible), its paged metadata upload, and the attention layer’s routing; before 2026-09-28 the layer read the lever alone and refused a varlen-admitted chunk-zero wave after admission, which failed every stream in it.