Split forward_only into forward_only_dispatch (no sync, no download)
and forward_only (sync + download). forward_encoder now calls the
dispatch-only path — it only needs h_t_d on device, stream-ordered.
Was: 2 × (raw_stream_sync + probs download) per step from the two
forward_encoder calls in step_with_lobsim. Each sync drained the
entire GPU pipeline while CPU did host staging for the next call.
GPU was 98.5% idle per nsys.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
107649c03b