ModelTranslator: base 贴图写 PNG 前做 linear->sRGB 编码

Blender Image.pixels 对 sRGB 贴图返回线性值，直接写 Non-Color PNG 会偏暗；
现按 Unity sRGB 导入预期编码 RGB，alpha（AO/透明）保持原值。附单元测试。

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

Tools/ModelTranslator/tests/test_bl_convert.py

@@ -0,0 +1,23 @@
+import os
+import sys
+import unittest
+
+import numpy as np
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+import bl_convert as bc
+
+
+class TestBaseTextureEncoding(unittest.TestCase):
+    def test_base_texture_rgb_is_encoded_as_srgb_and_alpha_is_preserved(self):
+        rgba = np.array([[[0.0, 0.18, 0.5, 0.37]]], dtype=np.float32)
+
+        encoded = bc.encode_base_texture_for_png(rgba)
+
+        expected_rgb = np.array([0.0, 0.461356, 0.735357], dtype=np.float32)
+        np.testing.assert_allclose(encoded[0, 0, :3], expected_rgb, rtol=0, atol=1e-5)
+        self.assertAlmostEqual(float(encoded[0, 0, 3]), 0.37, places=6)
+
+
+if __name__ == "__main__":
+    unittest.main()