Terminal: buy-average price per coin in Poinbase and Web3; drop the price line on the Poinbase card

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Decentralized.Domain/Wallet.cs

@@ -19,11 +19,30 @@ namespace Decentralized.Domain
         /// <summary>被执法标记。</summary>
         public bool Flagged;
 
+        /// <summary>买入记录：每种币买入的数量与花费（信用点等值），用来显示买入均价。卖出或转出时按比例减少。</summary>
+        public Dictionary<string, double> BuyQty = new Dictionary<string, double>(), BuyCost = new Dictionary<string, double>();
+
+        public void RecordBuy(string asset, double qty, double costCredits)
+        {
+            if (qty <= 0) return;
+            BuyQty[asset] = (BuyQty.TryGetValue(asset, out var q) ? q : 0) + qty;
+            BuyCost[asset] = (BuyCost.TryGetValue(asset, out var c) ? c : 0) + Math.Max(0, costCredits);
+        }
+
+        /// <summary>买入均价（信用点）；没有买入记录（例如全是挖出来的）返回 null。</summary>
+        public double? AvgBuy(string asset) => BuyQty.TryGetValue(asset, out var q) && q > 1e-9 ? BuyCost[asset] / q : (double?)null;
+
         public double Get(string asset) => Balances.TryGetValue(asset, out var v) ? v : 0;
 
         public void Add(string asset, double amount)
         {
             if (amount == 0) return;
+            if (amount < 0 && BuyQty.TryGetValue(asset, out var bq))
+            {
+                double before = Get(asset), keep = before > 1e-12 ? Math.Max(0, 1 + amount / before) : 0;
+                BuyQty[asset] = bq * keep; BuyCost[asset] *= keep;
+                if (BuyQty[asset] < 1e-9) { BuyQty.Remove(asset); BuyCost.Remove(asset); }
+            }
             double v = Get(asset) + amount;
             if (v < -1e-9) throw new InvalidOperationException($"{Address}: {asset} 余额不足");
             if (Math.Abs(v) < 1e-12) Balances.Remove(asset);