Function fa_seqs_eligible 

pub fn fa_seqs_eligible(t_kv: usize, head_dim: usize) -> bool

BATCHED-TICK increment 2 (2026-08-01): true iff a row at this t_kv would take the v4 eager arm in fa_decode_kvmod‘s dispatch — the exact precondition for the z-batched fa_decode_vec_q_seqs_v4 twin to reproduce its per-seq program bit-identically. Mirrors the kvmod predicates: vec on + above the vec floor + hd256 + inside the v4 window + the PRODUCTION v4 body (the noB3/stage phase probes are wrong-output) + the default flash module (no fp8-KV g-module). Callers must ALSO group rows on one fa_split_keys rung (the rows-twins’ straddle law) before batching.