feat: 修炼数值配置化，天灵根规则重构，更新主菜单背景

- GameConfig 新增 [cultivation] 修炼系统配置（基础速度/灵气浓度/资质及灵根倍率/天灵根加成）
- 天灵根改为仅天骄资质再掷（heavenly_root_chance=50%），移入 GameConfig 可配置
- Disciple 修炼倍率常量改为从 GameConfig 读取，新增灵根数量倍率方法
- 主菜单背景更换为"图片大门"，新增"宗门全景"资源，删除旧 BG1.png
- 同步更新弟子系统设计文档及新增修炼系统设计文档

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