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/generate_scene.py

@@ -116,6 +116,11 @@ TUFT_LIMIT_PER_LAWN = 100
 TUFT_TINT = (0.15, 0.52, 0.09)
 TUFT_TINT_FACTOR = 0.66
 
+# Tree styles. The two built from mesh batches live in this file; the rest are
+# vendored models handled by osmassets.tree, which owns that list. A model style
+# whose asset is missing falls back to "natural" rather than planting nothing.
+TREE_STYLES = frozenset(("natural", "procedural")) | frozenset(_tree.MODEL_STYLES)
+
 
 def cli_args():
     values = {"osm": None, "geojson": None, "output": None, "render": None,
@@ -143,8 +148,9 @@ def cli_args():
             values["office_overrides"] = set()
     else:
         values["office_overrides"] = set()
-    if values.get("tree_style") not in {"natural", "procedural", "polyhaven"}:
-        raise RuntimeError("--tree-style must be 'natural', 'procedural', or 'polyhaven'")
+    if values.get("tree_style") not in TREE_STYLES:
+        raise RuntimeError("--tree-style must be one of: "
+                           + ", ".join(sorted(TREE_STYLES)))
     return values
 
 
@@ -712,20 +718,25 @@ def build(args):
                                       height=parse_height(row_tags, 5.0))
         trees.extend(row_samples)
         row_tree_count += len(row_samples)
+    tree_style = args.get("tree_style")
+    tree_style_used = tree_style
     if trees:
-        tree_style = args.get("tree_style")
-        if tree_style == "polyhaven":
-            _tree.assemble(trees, props_c)
-        elif tree_style == "natural":
+        # A model style returns 0 when its vendored asset is missing; that drops
+        # through to "natural" so a clean checkout still gets trees.
+        placed = (_tree.assemble(trees, props_c, tree_style)
+                  if tree_style in _tree.MODEL_STYLES else 0)
+        if not placed:
+            tree_style_used = "procedural" if tree_style == "procedural" else "natural"
             tree_trunk = material_from_spec(catalog.MATERIALS["tree_trunk"])
-            add_natural_tree_instances(
-                trees, props_c, tree_trunk,
-                material_from_spec(catalog.MATERIALS["tree_crown_dark"]),
-                material_from_spec(catalog.MATERIALS["tree_crown_light"]))
-        else:
-            tree_trunk = material_from_spec(catalog.MATERIALS["tree_trunk"])
-            add_tree_batch(trees, props_c, tree_trunk,
-                           material_from_spec(catalog.MATERIALS["tree_crown"]))
+            if tree_style_used == "procedural":
+                add_tree_batch(
+                    trees, props_c, tree_trunk,
+                    material_from_spec(catalog.MATERIALS["tree_crown"]))
+            else:
+                add_natural_tree_instances(
+                    trees, props_c, tree_trunk,
+                    material_from_spec(catalog.MATERIALS["tree_crown_dark"]),
+                    material_from_spec(catalog.MATERIALS["tree_crown_light"]))
 
     for feature in point_features:
         if feature["tags"].get("amenity") != "fountain":
@@ -793,6 +804,9 @@ def build(args):
     scene["tree_node_count"] = individual_tree_count
     scene["tree_row_count"] = row_tree_count
     scene["tree_count"] = len(trees)
+    scene["tree_style"] = tree_style
+    # Differs from tree_style when a model style's asset was missing.
+    scene["tree_style_used"] = tree_style_used
     scene["road_feature_counts"] = json.dumps(road_counts, ensure_ascii=True)
 
     os.makedirs(os.path.dirname(args["output"]), exist_ok=True)
@@ -819,6 +833,7 @@ def build(args):
                                     "tree_nodes": individual_tree_count,
                                     "tree_row_instances": row_tree_count,
                                     "trees": len(trees),
+                                    "tree_style": tree_style_used,
                                     "road_features": road_counts}, ensure_ascii=True))