feat: 树渲染改用 apple/fattree 模型，修复 Cesium alpha 抠图丢失

tree_style 新增 apple（SpeedTree Red Delicious，4475 tris，alpha 抠图叶片
+ 法线贴图）和 fattree（低面数卡通树，2238 tris，实体几何）。

高度改为按模型自身包围盒归一化到目标高度，树根落在 z=0，不再用魔数；
每棵树按序号做确定性抖动（缩放 ±14%、黄金角偏航、±3° 倾斜），
tree_row 采样高度全部相同，不抖动就是同一棵树盖章 156 次。

移除 polyhaven 样式和 island_tree_01 资产（76MB）+ ingest_tree.py：
该样式 append 的 *_LOD1 并不是完整的树，枝干只有 0.41 单位高，叶片是
挂在原点下方的平面簇，本是给源文件几何节点散布用的碎片。

Cesium 侧修三处：

- make_export_material 把 Alpha 恒定写死为 1.0，叶片卡片整块导出，而
  SpeedTree 图集抠掉的区域是纯黑，在 Cesium 里就是黑色色块。
- Blender 4.2 起 glTF 导出不再读 blend_method（仍可写但已失效，写 CLIP
  读回来是 HASHED），改为从节点树推断 alpha 模式，alpha 直连 BSDF 会落到
  BLEND。新增 materials.link_alpha_clip() 构造导出器识别的
  1 - (alpha < cutoff) 结构，得到 alphaMode=MASK。
- 新增 alpha_dilated_image()：把不透明像素颜色向抠图区域外扩 8 圈，
  避免 mipmap 把黑色平均进叶缘。叶缘相邻的纯黑像素 10.3% → 0.6%。

另修两个既有 bug：

- 材质槽在 mesh 上，181 棵树共享一个 datablock，第一棵替换后其余会把结果
  再包一层，产生 Cesium Cesium Cesium... 的材质名；烘焙图缓存按材质名索引，
  每轮再嵌一份同样的贴图。GLB 22.46MB → 20.76MB，materials 270 → 23。
- shrub_02 从 glTF 带进来一个 Math 节点接在 Alpha 上，但其 JPEG 贴图 alpha
  全是 1.0，只判断「连了 Alpha」会误判为抠图材质。

模型资产为第三方素材，已 gitignore，缺失时回落到 natural；
来源见 assets/models/SOURCES.md。

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

blender/osmassets/materials.py

@@ -149,6 +149,52 @@ def make_textured_material(name, diffuse_file, normal_file, roughness,
     return material
 
 
+def link_alpha_clip(material, alpha_output, bsdf, cutoff=0.5):
+    """Wire a texture's alpha into `bsdf` as a hard cut-out.
+
+    The obvious wiring — alpha straight into the Alpha socket — is wrong for
+    anything destined for glTF.  Blender 4.2 stopped deriving a material's
+    alpha mode from `blend_method` (still writable, now a no-op: setting 'CLIP'
+    reads back 'HASHED') and made the exporter infer it from the node tree
+    instead.  It recognises exactly a few shapes; a bare link is not one of
+    them, and falls through to alphaMode=BLEND.  Foliage exported as BLEND
+    makes Cesium depth-sort thousands of leaf quads it cannot order correctly.
+
+    So build the shape the exporter looks for — `1 - (alpha < cutoff)` — which
+    it reads back as alphaMode=MASK with this cutoff.  EEVEE gets the same
+    thing for free: alpha is 0 or 1 by the time it reaches the BSDF, so the
+    viewport shows the crisp cut-out Cesium will, not a dithered approximation.
+
+    See the exporter's `detect_alpha_clip` in
+    scripts/addons_core/io_scene_gltf2/blender/exp/material/search_node_tree.py.
+    """
+    nodes = material.node_tree.nodes
+    links = material.node_tree.links
+
+    less_than = nodes.new("ShaderNodeMath")
+    less_than.operation = "LESS_THAN"
+    less_than.location = (-60, 320)
+    less_than.inputs[1].default_value = cutoff
+
+    invert = nodes.new("ShaderNodeMath")
+    invert.operation = "SUBTRACT"
+    invert.location = (110, 320)
+    invert.inputs[0].default_value = 1.0
+
+    links.new(alpha_output, less_than.inputs[0])
+    links.new(less_than.outputs["Value"], invert.inputs[1])
+    links.new(invert.outputs["Value"], bsdf.inputs["Alpha"])
+
+    # EEVEE Next takes its cut-out handling from surface_render_method, not
+    # from blend_method. 'DITHERED' still casts a leaf-shaped shadow;
+    # 'BLENDED' does not.
+    material.surface_render_method = "DITHERED"
+    material.alpha_threshold = cutoff
+    # Leaf cards are single-sided quads seen from both sides; culling
+    # backfaces would empty out half of every crown.
+    material.use_backface_culling = False
+
+
 def from_spec(spec):
     """Build a material from a `catalog.MATERIALS` entry."""
     if spec["kind"] == "textured":