Action space 81→108: direction(4) × magnitude(3) × order(3) × urgency(3).
Short(0), Hold(1), Long(2), Flat(3). Hold keeps current position with
zero transaction cost, giving the model a "do nothing" option.
Changes: trade_physics.cuh (Hold returns current_position), env_step
(Hold skips trade execution), action.rs (ExposureLevel::Hold variant),
config (branch_0_size 3→4), all match arms updated across 11 files.
Counterfactual mirror: Short↔Long, Hold↔Hold, Flat↔Flat.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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