feat: 添加 R_LOAD指令的实现

- 新增 RLoadCommand 类，实现 Command接口
- 该指令用于从局部变量表中加载引用对象，并将其推入操作数栈- 指令格式：R_LOAD <slot>
- 其中 <slot> 是局部变量表中的索引
- 执行过程包括解析索引、获取引用、推入栈顶和更新程序计数器

src/main/java/org/jcnc/snow/vm/commands/ref/control/RLoadCommand.java

@@ -0,0 +1,54 @@
+package org.jcnc.snow.vm.commands.ref.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;
+
+/**
+ * The {@code RLoadCommand} class implements the {@link Command} interface and represents the
+ * reference load instruction ({@code R_LOAD}) in the virtual machine.
+ *
+ * <p>
+ * This instruction loads a reference object from the current stack frame’s local variable store
+ * at the specified slot and pushes it onto the operand stack.
+ * </p>
+ *
+ * <p>Instruction format: {@code R_LOAD <slot>}</p>
+ * <ul>
+ *   <li>{@code <slot>}: The index in the local variable table to load the reference from.</li>
+ * </ul>
+ *
+ * <p>Behavior:</p>
+ * <ul>
+ *   <li>Parses the slot index from the instruction parameters.</li>
+ *   <li>Fetches the reference object stored at the specified slot in the current stack frame's local variable store.</li>
+ *   <li>Pushes the fetched reference onto the operand stack.</li>
+ *   <li>Increments the program counter to the next instruction.</li>
+ * </ul>
+ */
+public final class RLoadCommand implements Command {
+
+    /**
+     * Executes the {@code R_LOAD} instruction, loading a reference from the local variable table and pushing it onto the operand stack.
+     *
+     * @param parts The instruction parameters. {@code parts[0]} is the operator ("R_LOAD"), {@code parts[1]} is the slot index.
+     * @param pc The current program counter value, indicating the instruction address being executed.
+     * @param stack The operand stack manager. The loaded reference will be pushed onto this stack.
+     * @param lvs The local variable store. (Not used directly, as this command uses the store from the current stack frame.)
+     * @param cs The call stack manager. The reference will be loaded from the local variable store of the top stack frame.
+     * @return The next program counter value ({@code pc + 1}), pointing to the next instruction.
+     * @throws NumberFormatException if the slot parameter cannot be parsed as an integer.
+     */
+    @Override
+    public int execute(String[] parts, int pc,
+                       OperandStack stack,
+                       LocalVariableStore lvs,
+                       CallStack cs) {
+
+        int slot = Integer.parseInt(parts[1]);
+        Object v = cs.peekFrame().getLocalVariableStore().getVariable(slot);
+        stack.push(v);
+        return pc + 1;
+    }
+}