ModelTranslator: QR use ExactQuadCount, authoritative QR_ENGINE override

真机验证发现 ExactQuadCount=0 自适应在高细节硬表面模型上面数暴涨
(well1500 目标2500->11500 quads)并连带 seam 展开重叠爆炸;改用 exact=True
尊重目标数后 well1500 得 5940 面/591 UV岛(优于 collapse 745)。
find_qr_engine 改为权威覆盖:QR_ENGINE 无效即 None,不再静默自动发现,
使强制回退可测、覆盖契约成立。

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

Tools/ModelTranslator/tests/test_qr_bridge.py

@@ -83,6 +83,17 @@ class TestFindQrEngine(unittest.TestCase):
         os.environ["APPDATA"] = tempfile.mkdtemp()
         self.assertIsNone(qb.find_qr_engine())
 
+    def test_invalid_override_does_not_fall_through(self):
+        # QR_ENGINE 权威：显式设了但无效，即便 APPDATA 下装了真引擎也返回 None
+        root = tempfile.mkdtemp()
+        os.environ["QR_ENGINE"] = os.path.join(root, "nope.exe")
+        os.environ["APPDATA"] = root
+        eng_dir = os.path.join(root, "Blender Foundation", "Blender", "5.0",
+                               "scripts", "addons", "QuadRemesher", "Engine")
+        os.makedirs(eng_dir)
+        open(os.path.join(eng_dir, "xremesh.exe"), "w").close()
+        self.assertIsNone(qb.find_qr_engine())
+
     def test_globs_addon_path_picks_last(self):
         root = tempfile.mkdtemp()
         os.environ.pop("QR_ENGINE", None)