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/osmassets/tree.py

@@ -95,8 +95,33 @@ def load_tree_variants():
     for obj in imported:
         bpy.data.objects.remove(obj, do_unlink=True)
 
-    tris = sum(len(bm.loop_triangles) + len(lm.loop_triangles)
-               for bm, lm in variants)
-    print(f"Poly Haven tree variants loaded: {len(variants)} "
-          f"({tris} tris per instance)")
-    return variants
+def assemble(positions, collection):
+    """Place island_tree_01 instances at `positions`.
+
+    positions is a list of (x, y, height) tuples.  Returns the number of
+    trees placed (0 when the model is unavailable).
+    """
+    import math  # noqa — tree.py has no top-level math import by design
+
+    variants = load_tree_variants()
+    if not variants:
+        return 0
+
+    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)
+
+    return len(positions)