refactor: tree.assemble() 接管树构建，_add_polyhaven_trees 移除

把 polyhaven 树的实例化逻辑从 generate_scene.py 移到
osmassets/tree.py::assemble()，与 water/grass/scrub 的要素
注册表模式对齐。

generate_scene.py 现在只有一个 _assemble_building 残留（因为需要
make_prism / add_roof / add_wall_panel 这些 mesh 构建函数）。
后续 P2c 会把 building 也搬出去。

parity: control-1 vs p2b-unify = PARITY OK (两区域)
smoke: tree_style=polyhaven SCENE_DONE (181棵树, exit 0)
tests: 42/42 OK

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

blender/generate_scene.py

@@ -91,40 +91,6 @@ def _assemble_building(ring, way_id, tag, args, buildings_c, building_mats):
                          building_mats, buildings_c)
     return 1, int(industrial), ring
 
-
-def _add_polyhaven_trees(positions, collection):
-    """Scatter island_tree_01 instances at `positions`.
-
-    positions is a list of (x, y, height) tuples as gathered by the two
-    tree-collecting loops (point nodes + tree_row samples).
-    """
-    variants = _tree.load_tree_variants()
-    if not variants:
-        # Fall back to procedural so a missing model is not a hard build error.
-        print("Poly Haven tree model unavailable; switching to procedural trees")
-        add_tree_batch(positions, collection,
-                       material_from_spec(catalog.MATERIALS["tree_trunk"]),
-                       material_from_spec(catalog.MATERIALS["tree_crown"]))
-        return
-
-    for index, (x, y, height) in enumerate(positions):
-        branch_mesh, leaf_mesh = variants[index % len(variants)]
-        target = max(4.0, height)
-        factor = target / 3.2
-
-        b_obj = bpy.data.objects.new(f"Tree_PH_{index:03d}_Branch", branch_mesh)
-        b_obj.location = (x, y, 0.0)
-        b_obj.scale = (factor, factor, factor)
-        b_obj.rotation_euler = (0, 0, (index * 1.61803398875) % 1.0 * math.tau)
-        collection.objects.link(b_obj)
-
-        l_obj = bpy.data.objects.new(f"Tree_PH_{index:03d}_Leaf", leaf_mesh)
-        l_obj.location = (x, y, 0.0)
-        l_obj.scale = (factor, factor, factor)
-        l_obj.rotation_euler = (0, 0, (index * 1.61803398875) % 1.0 * math.tau)
-        collection.objects.link(l_obj)
-
-
 MODEL_ROOT = os.path.abspath(os.path.join(
     os.path.dirname(__file__), "..", "assets", "models", "polyhaven"
 ))
@@ -749,7 +715,7 @@ def build(args):
     if trees:
         tree_style = args.get("tree_style")
         if tree_style == "polyhaven":
-            _add_polyhaven_trees(trees, props_c)
+            _tree.assemble(trees, props_c)
         elif tree_style == "natural":
             tree_trunk = material_from_spec(catalog.MATERIALS["tree_trunk"])
             add_natural_tree_instances(