ModelTranslator: uvgate 回退标注改前缀匹配——兼容 +uvpm 后缀

- 新增 seam_fallback_label 纯函数：前缀匹配转换 seam* → seam_fallback*，保留后缀
- main() 调用改为函数式，避免等值判断失效
- 测试覆盖：plain seam、seam+uvpm、smart_fallback 透传

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Tools/ModelTranslator/tests/test_bl_uvgate.py

@@ -0,0 +1,23 @@
+import os
+import sys
+import unittest
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+import bl_uvgate as bg
+
+
+class TestSeamFallbackLabel(unittest.TestCase):
+    def test_plain_seam(self):
+        self.assertEqual(bg.seam_fallback_label("seam"), "seam_fallback")
+
+    def test_seam_with_uvpm_suffix(self):
+        self.assertEqual(bg.seam_fallback_label("seam+uvpm"), "seam_fallback+uvpm")
+
+    def test_smart_fallback_untouched(self):
+        self.assertEqual(bg.seam_fallback_label("smart_fallback"), "smart_fallback")
+        self.assertEqual(bg.seam_fallback_label("smart_fallback+uvpm"),
+                         "smart_fallback+uvpm")
+
+
+if __name__ == "__main__":
+    unittest.main()