ModelTranslator: QR watertightness gate + quad default

根因(真机排查):QuadRemesher 引擎在激进减面下非确定性地从封闭流形
输入打出孔洞/非流形边(实测同一输入约 85% 次数破碎),即"破碎面"。

修复:QR 导回后比对输入拓扑,若比输入新增开边(孔洞)/非流形边即判破损、
弃用 QR 回退 collapse(topology_acceptable 纯策略+单测;_edge_defects 计数)。
顺带修复水密门在导入后拒绝时未恢复 obj 选中态,导致 collapse 回退时
export use_selection 选不到对象、只导出 4KB 空 FBX 的 bug。

含用户改动:--reducer 默认改为 quad(配合水密门,破碎自动回退,安全),
及其 CLI 默认值测试。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Tools/ModelTranslator/tests/test_model_decimate_cli.py

@@ -0,0 +1,39 @@
+import os
+import sys
+import unittest
+from unittest import mock
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+import model_decimate
+
+
+class TestModelDecimateCli(unittest.TestCase):
+    def test_default_reducer_is_quad(self):
+        summary = {
+            "src": "store.fbx",
+            "tris_before": 49966,
+            "tris_after": 3000,
+            "target": 3000,
+            "reducer": "quad",
+            "uv": {
+                "mode": "seam",
+                "islands": 1,
+                "fill": 1.0,
+                "flipped": 0.0,
+                "overlap": 0.0,
+            },
+            "warnings": [],
+        }
+        argv = ["model_decimate.py", "store.fbx", "--tris", "3000"]
+
+        with mock.patch.object(sys, "argv", argv), \
+             mock.patch.object(model_decimate, "find_blender", return_value="blender"), \
+             mock.patch.object(model_decimate, "run_blender_script", return_value=summary) as run:
+            model_decimate.main()
+
+        args = run.call_args.args[2]
+        self.assertEqual(args[-1], "quad")
+
+
+if __name__ == "__main__":
+    unittest.main()

Tools/ModelTranslator/tests/test_qr_bridge.py

@@ -116,5 +116,30 @@ class TestFindQrEngine(unittest.TestCase):
         self.assertIn(os.path.join("5.0", "scripts"), chosen)
 
 
+class TestTopologyAcceptable(unittest.TestCase):
+    def test_closed_source_clean_qr_ok(self):
+        # 封闭输入(0/0) + 水密 QR(0/0) -> 接受
+        self.assertTrue(qb.topology_acceptable(0, 0, 0, 0))
+
+    def test_closed_source_holey_qr_rejected(self):
+        # 封闭输入被 QR 打出开边 -> 拒绝
+        self.assertFalse(qb.topology_acceptable(0, 0, 8, 0))
+
+    def test_closed_source_nonmanifold_qr_rejected(self):
+        self.assertFalse(qb.topology_acceptable(0, 0, 0, 1))
+
+    def test_open_shell_source_same_ok(self):
+        # 输入本身是开放壳(10 开边)，QR 未新增 -> 接受
+        self.assertTrue(qb.topology_acceptable(10, 0, 10, 0))
+
+    def test_open_shell_source_worse_rejected(self):
+        # QR 比输入引入更多开边 -> 拒绝
+        self.assertFalse(qb.topology_acceptable(10, 0, 12, 0))
+
+    def test_qr_fewer_defects_ok(self):
+        # QR 输出反而更少缺陷（不劣于输入）-> 接受
+        self.assertTrue(qb.topology_acceptable(10, 2, 8, 1))
+
+
 if __name__ == "__main__":
     unittest.main()