feat: CraftBank Folia 1.21.8 经济与银行核心插件初始实现

实现完整的现金/钱包、银行卡、支票、活期储蓄与定期存款系统，
实现并以 Highest 优先级注册 Vault Economy 接口，接入 PlaceholderAPI。
全程遵循 Folia 调度模型（AsyncScheduler / 实体区域线程），
数据缓存线程安全，支票兑现与定期领取做防刷处理。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/com/craftbank/hooks/CraftBankExpansion.java

@@ -0,0 +1,64 @@
+package com.craftbank.hooks;
+
+import com.craftbank.CraftBank;
+import com.craftbank.model.PlayerAccount;
+import me.clip.placeholderapi.expansion.PlaceholderExpansion;
+import org.bukkit.OfflinePlayer;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * PlaceholderAPI 扩展, 提供:
+ * <ul>
+ *   <li>%craftbank_cash% —— 现金余额</li>
+ *   <li>%craftbank_bank_saving% —— 活期余额</li>
+ *   <li>%craftbank_cash_formatted% —— 带货币符号的现金</li>
+ *   <li>%craftbank_bank_saving_formatted% —— 带货币符号的活期</li>
+ * </ul>
+ */
+public class CraftBankExpansion extends PlaceholderExpansion {
+
+    private final CraftBank plugin;
+
+    public CraftBankExpansion(CraftBank plugin) {
+        this.plugin = plugin;
+    }
+
+    @Override
+    public @NotNull String getIdentifier() {
+        return "craftbank";
+    }
+
+    @Override
+    public @NotNull String getAuthor() {
+        return "CraftBank";
+    }
+
+    @Override
+    public @NotNull String getVersion() {
+        return plugin.getDescription().getVersion();
+    }
+
+    @Override
+    public boolean persist() {
+        return true;
+    }
+
+    @Override
+    public String onRequest(OfflinePlayer player, @NotNull String params) {
+        if (player == null) {
+            return "";
+        }
+        // 占位符在主线程被解析: 仅读取已缓存数据, 不触发阻塞式数据库查询。
+        PlayerAccount account = plugin.getEconomyManager().getCached(player.getUniqueId());
+        double cash = account != null ? account.getCash() : 0.0;
+        double saving = account != null ? account.getBankSaving() : 0.0;
+
+        return switch (params.toLowerCase()) {
+            case "cash" -> plugin.getEconomyManager().formatNumber(cash);
+            case "cash_formatted" -> plugin.getEconomyManager().format(cash);
+            case "bank_saving" -> plugin.getEconomyManager().formatNumber(saving);
+            case "bank_saving_formatted" -> plugin.getEconomyManager().format(saving);
+            default -> null;
+        };
+    }
+}