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TestFSMOccupancyAndRollback covers the invariants that prevent orphaned exchange orders — the production failure mode where multiple positions accumulated because slot state wasn't rolled back on submit failure: - ENTRY_WORKING blocks new ENTER (different trade_id → SLOT_BUSY) - POSITION_OPEN blocks new ENTER - venue.submit raise → synthetic REJECTED → FSM back to IDLE - After rollback slot immediately reusable - N consecutive submit failures never strand the slot - submit-fail then success → exactly 1 position, not N - 20 rapid enter→exit cycles leave no residual state - EXIT on IDLE always rejected (no phantom closes) - 5 assets, 1 slot → only first accepted, rest SLOT_BUSY Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>