feat: 接入 Poly Haven island_tree_01 真实扫描树模型

tree_style 新增 'polyhaven' 选项，用 wm.append 从 vendored .blend
加载 LOD1 网格（4 种 branch+leaf 组合），通过实例化放置场景中的树。

每棵树约 625 tris / 2 meshes (branch+leaf)，比旧 procedural tree
(~400 tris) 多了约 60%，但换来真实树形和 PBR 贴图，树叶有 alpha
透明度，在 Cesium 远距离比纯几何球体更可读。

纹理从 ~/Downloads 手动下载（Poly Haven CDN 的 API URL 不可用），
存放在 assets/models/polyhaven/island_tree_01/textures/（不入 git）。

默认 tree_style 仍为 'natural'，parity 保持不变。

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

blender/osmassets/tree.py

@@ -0,0 +1,102 @@
+"""Import Poly Haven island_tree_01 as instanced-tree asset.
+
+This is the analog of load_tuft_variants() for trees.  It appends only the
+LOD1 meshes from the vendored .blend, which automatically pulls in their
+material definitions and texture images.  The top-level object parenting
+(branch and leaf parts share one origin) is handled by the caller who
+instances them as a single logical tree.
+"""
+
+import os
+
+import bpy
+
+
+TREE_BLEND = os.path.join(
+    os.path.dirname(os.path.abspath(__file__)), "..", "..",
+    "assets", "models", "polyhaven", "island_tree_01",
+    "island_tree_01_1k.blend",
+)
+
+# Branch / leaf LOD1 pairings for the four variants.
+# Each pairing is a single logical tree that the caller instances as one.
+VARIANT_PAIRS = [("a", "b"), ("b", "a"), ("c", "a"), ("d", "b")]
+
+
+def load_tree_variants():
+    """Import the vendored Poly Haven tree and return meshes ready to instance.
+
+    Returns a list of (branch_mesh, leaf_mesh) tuples, one per variant.  The
+    meshes share the material datablocks that were appended alongside them, so
+    the textures and shader nodes are wired up automatically.
+
+    Returns [] when the source .blend is absent so a clean checkout still
+    builds.  The caller falls back to procedural trees.
+    """
+    if not os.path.exists(TREE_BLEND):
+        return []
+
+    before_objects = set(bpy.data.objects)
+    before_meshes = set(bpy.data.meshes)
+
+    # Gather the object names we need from the library.
+    wanted = []
+    for branch_id, leaf_id in VARIANT_PAIRS:
+        wanted.append(f"island_tree_01_branches_{branch_id}_LOD1")
+        wanted.append(f"island_tree_01_leaves_{leaf_id}_LOD1")
+
+    # bpy.ops.wm.append cannot handle a list of files in a single call
+    # reliably (it may crash when objects in the same library share data
+    # that was already linked by an earlier append in the same batch), so
+    # we append one at a time and skip the "already linked" warning.
+    for name in wanted:
+        try:
+            bpy.ops.wm.append(
+                filepath=TREE_BLEND + "/Object/" + name,
+                directory=TREE_BLEND + "/Object/",
+                files=[{"name": name}],
+                link=False,
+            )
+        except RuntimeError:
+            # Object already linked by an earlier append of a sibling mesh
+            # that shared materials; this is expected and harmless.
+            pass
+
+    imported = [obj for obj in set(bpy.data.objects) - before_objects
+                if obj.type == "MESH"]
+
+    # Pair branch + leaf into variants.  We append in the order above so
+    # a simple zip works.
+    variant_meshes = []
+    for i in range(0, len(imported), 2):
+        if i + 1 >= len(imported):
+            break
+        # The import order is interleaved (branch, leaf, branch, leaf, …)
+        # because `wanted` alternated.  Sort by name to be safe.
+        pass
+
+    # Rebuild the pairs from the imported object names.
+    variants = []
+    for branch_id, leaf_id in VARIANT_PAIRS:
+        b_obj = bpy.data.objects.get(
+            f"island_tree_01_branches_{branch_id}_LOD1")
+        l_obj = bpy.data.objects.get(
+            f"island_tree_01_leaves_{leaf_id}_LOD1")
+        if b_obj and l_obj:
+            # Copy the meshes so they survive object removal.
+            b_mesh = b_obj.data.copy()
+            b_mesh.name = "PolyHavenTree_Branch_" + branch_id
+            b_mesh.use_fake_user = True
+            l_mesh = l_obj.data.copy()
+            l_mesh.name = "PolyHavenTree_Leaf_" + leaf_id
+            l_mesh.use_fake_user = True
+            variants.append((b_mesh, l_mesh))
+
+    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