feat: LityPy - Python scripting library for lity3d engine

- Vec3, Color, Transform, Entity, Scene with decorator API
- Components: MeshRenderer, RigidBody, Collider, Script
- Physics helpers, Tween animations, Camera follow/orbit
- Game templates: platformer, RPG, FPS
- Scene save/load (JSON), export to .ly
- 12 unit tests passing, 89-entity RPG demo verified

Demo/demo_rpg.py

@@ -0,0 +1,105 @@
+"""LityPy Demo - 3D 开放世界 RPG"""
+from lity import *
+
+scene = Scene("LityPy RPG Demo", 1280, 720)
+camera = Camera()
+
+# 地面
+ground = create_ground(scene, 60, Color(0.25, 0.5, 0.2))
+
+# 玩家
+player = scene.create("hero", "cube", pos=Vec3(0, 0.5, 0), color=Color.GREEN)
+player.speed = 5.0
+player.add_rigidbody()
+player.add_collider(size=Vec3(0.5, 1, 0.5))
+
+# 村庄建筑
+walls = [
+    create_building(scene, -5, -5, 2, 2, 2, Color.RED, Color.GRAY),
+    create_building(scene, 3, -6, 2.5, 2.5, 2, Color.BLUE, Color.GRAY),
+    create_building(scene, -2, -8, 1.8, 1.8, 1.8, Color.YELLOW, Color(0.45, 0.28, 0.15)),
+    create_building(scene, 6, -3, 2, 3, 2, Color.ORANGE, Color.GRAY),
+]
+
+# NPC
+npc1 = scene.create("merchant", "cube", pos=Vec3(1, 0.4, -4), color=Color.CYAN, tags=["npc"])
+npc1.dialog = "欢迎来到村庄！"
+npc2 = scene.create("guard", "cube", pos=Vec3(-4, 0.4, -3), color=Color.MAGENTA, tags=["npc"])
+npc2.dialog = "小心森林里的怪物！"
+
+# 树木
+for i in range(25):
+    x, z = rand(-25, 25), rand(-25, 25)
+    if abs(x) < 8 and abs(z) < 10: continue
+    create_tree(scene, x, z)
+
+# 岩石
+for i in range(10):
+    x, z = rand(-25, 25), rand(-25, 25)
+    scene.create(f"rock_{i}", "sphere", pos=Vec3(x, 0.3, z),
+                 scale=Vec3(rand(0.3, 0.8), rand(0.2, 0.5), rand(0.3, 0.8)),
+                 color=Color(0.5, 0.5, 0.5))
+
+# 花
+for i in range(15):
+    x, z = rand(-10, 10), rand(-10, 10)
+    c = Color(rand(0.6, 1), rand(0.1, 0.5), rand(0.1, 0.5))
+    scene.create(f"flower_{i}", "sphere", pos=Vec3(x, 0.15, z),
+                 scale=Vec3(0.1, 0.1, 0.1), color=c)
+
+# 水塘
+scene.create("water", "plane", pos=Vec3(-8, 0.05, -3),
+             scale=Vec3(4, 1, 4), color=Color(0.15, 0.3, 0.6, 0.8))
+
+# 远山
+for i, (x, z, sx, sy, sz) in enumerate([
+    (-20, 15, 8, 3, 8), (22, 12, 10, 4, 10), (0, 25, 12, 3, 8)
+]):
+    scene.create(f"hill_{i}", "sphere", pos=Vec3(x, -0.5, z),
+                 scale=Vec3(sx, sy, sz), color=Color(0.2, 0.4, 0.18))
+
+# 敌人
+enemies = []
+for i in range(8):
+    x, z = rand(-20, 20), rand(5, 20)
+    e = scene.create(f"enemy_{i}", "sphere", pos=Vec3(x, 0.5, z),
+                     color=Color(0.8, 0.2, 0.1), tags=["enemy"])
+    e.hp = 100
+    e.speed = 2.0
+    e.patrol_center = Vec3(x, 0.5, z)
+    enemies.append(e)
+
+# ===== 游戏逻辑 =====
+score = 0
+hp = 100
+
+@scene.on_frame
+def update(dt):
+    global score, hp
+    camera.follow(player, offset=Vec3(0, 4, -8), smooth=3.0 * dt)
+    
+    for e in enemies:
+        d = e.pos.distance(player.pos)
+        if d < 15:
+            to_player = (player.pos - e.pos).normalize()
+            e.pos = e.pos + to_player * e.speed * dt
+            e.rot = Vec3(0, math.atan2(to_player.x, to_player.z), 0)
+        if d < 8:
+            e.pos = e.pos.lerp(e.patrol_center, dt * 0.5)
+
+# 导出场景
+export_scene_ly(scene, "rpg_scene.ly")
+scene.save("rpg_scene.json")
+
+print(f"\n=== LityPy RPG Demo ===")
+print(f"场景: {len(scene.entities)} 个实体")
+print(f"玩家: {player.pos}")
+print(f"敌人: {len(enemies)} 个")
+print(f"建筑: {len(walls)} 栋")
+print(f"\n特性展示:")
+print(f"  Vec3: {Vec3(1,2,3) + Vec3(4,5,6)}")
+print(f"  Color.from_hex: {Color.from_hex('#FF5500')}")
+print(f"  lerp: {lerp(0, 10, 0.3)}")
+print(f"  rand: {rand(0, 100):.2f}")
+print(f"  sin: {math.sin(time_s()):.4f}")
+print(f"\n引擎 API 已就绪，需要编译 C 端集成。")