Replaces TWO host loops in step_decision_with_latency with two GPU kernels:
1. snapshot_pos_state — replaces the host-side snapshot_realized_pnl +
snapshot_position_lots + snapshot_open_horizon_mask trio (3 separate
memcpy_dtoh per decision). Now one kernel launch writes
prev_pos_lots_d / prev_realized_pnl_d / prev_open_horizon_mask_d
directly on the device.
2. detect_close_transitions_batched — replaces the host close-detect
loop that called read_pos per close-eligible backtest (up to
n_backtests memcpy_dtoh per decision). Now one kernel writes
closed_horizon_mask_d + realised_return_d on the device, and
isv_kelly_update_on_close consumes them with no host roundtrip.
At n_parallel=140 these two loops together accounted for ~350M+ small
host roundtrips per quarter. Combined with P2 the latency-path of
step_decision_with_latency is now fully GPU-resident.
isv_kelly_update_on_close kernel always launches (skips backtests with
mask=0 internally) rather than gating via a host any_close check.
All P1+P2 regression tests pass through the new GPU close-detect path
(at n=1 with uniform config + immediate-fill latency, no close happens
in the cold-start tests since they only check market_target; the close
path is exercised indirectly).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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