Remove unnecessary stream.synchronize() from:
1. step_fill_from_market_targets (lobsim): stream ordering handles
the dependency — step_pnl_track launches on the same stream
2. step_pnl_track (lobsim): downstream kernels read pos_d via
stream ordering, no host read needed per step
Defer pos_fraction readback by one step (same pattern as loss
readback) — eliminates the third 7ms sync. pos_fraction is a
dataset-level statistic that barely changes step to step.
nsys: 3.0 → 1.0 slow syncs/step. Sync time: 21.8 → 9.65 ms/step.
The remaining sync is the training graph event wait (actual compute).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
6b89dbfcb8