run_nan_checks_post_backward now fires a single fused kernel launch
covering slots 24-35 in 12 blocks. Reduces graph-capture overhead from
8 launches to 1 per step. Slots 33-35 inline checks at backward
orchestration sites (gpu_dqn_trainer.rs:18580/:18601/:6939) unchanged
— multi-point localization preserved.
Method signature simplified — IQN pointers no longer per-step args.
populate_nan_check_meta (called once at construction in fused_training.rs)
baked Option<u64> nullity into the metadata buffer entries; the fused
kernel's null-pointer guard handles slots 27/28 when IQN inactive
without per-step branching at the Rust caller.
Both call sites in fused_training.rs (ungraphed + graph-captured paths)
updated together per feedback_no_partial_refactor.
This is the F0 regression fix — Phase B's per-step kernel-launch
overhead was the F0 cause across 3 SP1 smokes (F0 ~ 35 vs baseline
55.87). Gate 1 smoke (Task A6) validates F0 >= 53.08.
b5a064f6ca