The warmup step set graph_warmup_done=true but never called
begin_capture; the end_capture check then fired prematurely, causing
CUDA_ERROR_ILLEGAL_STATE. Fixed by tracking a local `capturing_prefill`
/ `capturing_postfill` flag that's true only when begin_capture ran.
Also adds Graph A2 (post-snapshot) capture for 7 kernels:
session_risk → min_hold → trail_decay → trail_mutate →
trail_stop → position_heat → actions_to_market_targets.
Removes unused buf_len variable.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
2c9aefe19e