fix: initialize live2d in PixiStage constructor

[HardyNLee]

介绍

将初始化 Live 2D 的函数放在 PixiStage 的构造函数中
支持设置旧版表情混合回退

[gemini-code-assist]

Summary of Changes

Hello @HardyNLee, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求解决了Live2D初始化中的一个关键问题，通过将initLive2D函数的调用从Live2DCore构造函数移动到PixiStage构造函数。此前，直接在Live2DCore构造函数中初始化会导致加载失败。此修复确保Live2D在适当的阶段进行初始化。此外，此PR还引入了对旧版表情混合模式的支持，通过新的配置选项为Live2D表情处理提供了更大的灵活性。

Highlights

• Live2D初始化修复: Live2D.initLive2D()的调用已从Live2DCore构造函数移动到PixiStage构造函数，解决了Live2D在其自身核心类中直接初始化时加载失败的问题。
• 支持旧版表情混合模式: Live2DCore中新增了legacyExpressionBlendMode属性和相关逻辑，实现了Live2D模型旧版表情混合行为的动态配置。此设置现在可以通过新命令进行控制。
• Live2DCore重构: Live2DCore类已进行重构，将其构造函数与initLive2D调用解耦，并正确管理新的legacyExpressionBlendMode设置，确保在Live2D可用后应用配置。

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[gemini-code-assist]

Code Review

本次 PR 将 Live2D 的初始化逻辑从 Live2DCore 的构造函数中移至 PixiStage 的构造函数中异步调用，解决了之前提到的加载失败问题。同时，增加了对 legacyExpressionBlendMode 配置的支持，允许在游戏配置中进行设置。

整体修改方向正确，代码实现清晰。我只发现了一个小的代码冗余问题，并已在文件中提出了具体的修改建议。修复后即可合并。

[gemini-code-assist]

这里的 res === true 判断是多余的。在前面的代码中（第 47-49 行），res 已经被转换成布尔值了。为了保持代码一致性和简洁性，建议直接使用 res。

与此文件中的 Enable_Appreciation 的处理方式 dispatch(setEnableAppreciationMode(res)); 保持一致，可以提高代码的可读性。

建议修改为：

if (command === 'Legacy_Expression_Blend_Mode') {
  Live2D.legacyExpressionBlendMode = res;
}

Suggested change

	if (command === 'Legacy_Expression_Blend_Mode') {
	Live2D.legacyExpressionBlendMode = res === true;
	}
	if (command === 'Legacy_Expression_Blend_Mode') {
	Live2D.legacyExpressionBlendMode = res;
	}