predictor: static graphs replayed with a single id upload per step

The 15 predictor flavors build and allocate once at load, positions,
kv rows, and mask baked as never freed graph outputs, and replay
directly on the backend. The prefill slices the last position before
lm_head so every flavor reads one logits row at offset zero. Replaces
the per step graph rebuild, sched allocation, and debug prints.