v2.0.2 — Clean up empty Redis depth levels

When a fill or cancellation empties a price level, Mengine removes the Redis depth hash field and sorted-set member atomically.

Regression coverage verifies cleanup after fills, full cancellations, and partial cancellations that remove the entire remainder. It also verifies that placing a new order at the same price restores the correct depth.

Existing zero-volume fields are cleaned up when their price level goes through this path.