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

assets/sprites/ui/图片大门.png.import

@@ -2,16 +2,16 @@
 
 importer="texture"
 type="CompressedTexture2D"
-uid="uid://caeqoc6wx8toy"
-path="res://.godot/imported/BG1.png-6bf4b9c36da6bbdf71bd2c8203f4cb53.ctex"
+uid="uid://c6bthhmao5fyh"
+path="res://.godot/imported/图片大门.png-03ea66f8e1eb82ba7952374321f00064.ctex"
 metadata={
 "vram_texture": false
 }
 
 [deps]
 
-source_file="res://assets/sprites/ui/BG1.png"
-dest_files=["res://.godot/imported/BG1.png-6bf4b9c36da6bbdf71bd2c8203f4cb53.ctex"]
+source_file="res://assets/sprites/ui/图片大门.png"
+dest_files=["res://.godot/imported/图片大门.png-03ea66f8e1eb82ba7952374321f00064.ctex"]
 
 [params]
 

assets/sprites/ui/宗门全景.png.import

@@ -0,0 +1,40 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://cewfc3vetgbfx"
+path="res://.godot/imported/宗门全景.png-d969df84bf06c400df6e36c494864bbe.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://assets/sprites/ui/宗门全景.png"
+dest_files=["res://.godot/imported/宗门全景.png-d969df84bf06c400df6e36c494864bbe.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1