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SafeMul and caclAVGPrice compute with big.Int and then silently truncate the result with Int64(). For a large amount/price combination the wrapped value can be a small positive number, so a malicious passive buy order only freezes a tiny amount and the matching engine pays the taker seller the wrapped value while transferring the full token amount. The average price bookkeeping is corrupted the same way. - SafeMul/caclAVGPrice now return ErrOverflow when the result does not fit in int64. - Order creation, frozen remainder, matching settlement and average-price updates reject the overflow behind ForkFixSafeMulOverflow. - Pre-fork behavior is unchanged (the truncated value is still used) so the fix can be activated at a fork height.
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问题
SafeMul用 big.Int 计算后再Int64()截断，结果超出 int64 时静默回绕（可能变成一个小正数）且上层不感知。攻击：挂一笔被动买单，
Amount=28999941890838049、Price=1e16（均在CheckAmount/CheckPrice允许范围），名义成本约为 2.9e24，截断后恰好为 256，买单只冻结 256 个单位。taker 卖单卖出数量不小于该挂单数量时，matchModel用SafeMul(matched, 1e16, 1e8)计算付款同样回绕成 256，卖方交出大量 token 只收到 0.00000256 枚。caclAVGPrice也会同样截断，损坏订单均价/市场深度。修复
SafeMul、caclAVGPrice改为返回(int64, error)，!IsInt64()时返回ErrOverflow。ForkFixSafeMulOverflow，在下单余额检查、撤单、未成交冻结、撮合付款、均价计算所有路径分叉后拒绝溢出；分叉前仍沿用旧的截断值，保证历史区块可重放。CalcActualCost调用，并新增TestSafeMul溢出用例。验证
go build ./plugin/dapp/exchange/...通过go test ./plugin/dapp/exchange/executor/ -count=1通过