feat: 添加 SyscallCommand 类实现系统调用功能

- 新增 SyscallCommand 类，用于处理系统调用命令（opcode =0x0401）

src/main/java/org/jcnc/snow/vm/commands/system/control/SyscallCommand.java

@@ -0,0 +1,72 @@
+package org.jcnc.snow.vm.commands.system.control;
+
+import org.jcnc.snow.vm.interfaces.Command;
+import org.jcnc.snow.vm.module.CallStack;
+import org.jcnc.snow.vm.module.LocalVariableStore;
+import org.jcnc.snow.vm.module.OperandStack;
+
+import java.util.Arrays;
+
+/**
+ * SyscallCommand ―― 统一的系统调用入口（opcode = 0x0401）。
+ *
+ * <p>当前支持子命令：
+ * <ul>
+ *   <li><b>PRINT</b>   —— 打印，不换行</li>
+ *   <li><b>PRINTLN</b> —— 打印并换行</li>
+ * </ul>
+ *
+ * <p>用法示例（VM 指令）：</p>
+ * <pre>
+ *   1025 PRINT "Hello, Snow!"
+ *   I_PUSH 42
+ *   1025 PRINTLN
+ * </pre>
+ */
+public class SyscallCommand implements Command {
+
+    @Override
+    public int execute(String[] parts, int currentPC,
+                       OperandStack operandStack,
+                       LocalVariableStore localVariableStore,
+                       CallStack callStack) {
+
+        if (parts.length < 2)
+            throw new IllegalArgumentException("SYSCALL requires a sub-command");
+
+        String subCmd = parts[1].toUpperCase();
+
+        switch (subCmd) {
+            case "PRINT":
+            case "PRINTLN": {
+                boolean newline = subCmd.equals("PRINTLN");
+                String output;
+
+                // 指令里直接带字符串常量
+                if (parts.length > 2) {
+                    output = String.join(" ", Arrays.copyOfRange(parts, 2, parts.length));
+                    if (output.length() >= 2 &&
+                        output.startsWith("\"") &&
+                        output.endsWith("\"")) {
+                        output = output.substring(1, output.length() - 1); // 去掉首尾引号
+                    }
+                }
+                // 没带常量，则弹栈打印
+                else {
+                    Object value = operandStack.pop();
+                    output = String.valueOf(value);
+                }
+
+                if (newline) System.out.println(output);
+                else          System.out.print(output);
+                break;
+            }
+
+            default:
+                throw new UnsupportedOperationException("Unsupported SYSCALL: " + subCmd);
+        }
+
+        // 下一条指令
+        return currentPC + 1;
+    }
+}