imgcodecs: fix SunRaster IMREAD_GRAYSCALE returning black image

SunRasterDecoder::readData() only called CvtPaletteToGray() when the
file contained a color palette (m_maptype == RMT_EQUAL_RGB). But
SunRasterEncoder always writes RMT_NONE (no palette), so gray_palette[]
stayed zero-initialized and FillGrayRow8() mapped every pixel to 0.

Fix: call CvtPaletteToGray() whenever reading as grayscale, regardless
of palette type. readHeader() already fills m_palette correctly via
FillGrayPalette() for the RMT_NONE case.

Add regression test Imgcodecs_SunRaster.imread_grayscale_roundtrip
that writes a full 0-255 ramp, reads it back with IMREAD_GRAYSCALE,
and checks NORM_INF == 0. Without the fix, NORM_INF == 255 (all black).
Remove the "broken (black result)" skip for ras in read_write_GRAYSCALE.

modules/imgcodecs/src/grfmt_sunras.cpp

@@ -159,7 +159,7 @@ bool  SunRasterDecoder::readData( Mat& img )
     AutoBuffer<uchar> _src(src_pitch + 32);
     uchar* src = _src.data();
 
-    if( !color && m_maptype == RMT_EQUAL_RGB )
+    if( !color )
         CvtPaletteToGray( m_palette, gray_palette, 1 << m_bpp );
 
     try

modules/imgcodecs/test/test_read_write.cpp

@@ -264,7 +264,6 @@ TEST_P(Imgcodecs_Image, read_write_GRAYSCALE)
 
     if (false
         || ext == "ppm"  // grayscale is not implemented
-        || ext == "ras"  // broken (black result)
     )
         throw SkipTestException("GRAYSCALE mode is not supported");
 
@@ -291,6 +290,38 @@ TEST_P(Imgcodecs_Image, read_write_GRAYSCALE)
 
 INSTANTIATE_TEST_CASE_P(imgcodecs, Imgcodecs_Image, testing::ValuesIn(exts));
 
+#ifdef HAVE_IMGCODEC_SUNRASTER
+// Regression test: imread(file, IMREAD_GRAYSCALE) for a .ras file written by
+// imwrite() used to return an all-black image because gray_palette[] was never
+// populated when the file has no color palette (RMT_NONE, which is what
+// SunRasterEncoder always writes).
+TEST(Imgcodecs_SunRaster, imread_grayscale_roundtrip)
+{
+    // Build a 16x16 ramp covering all 256 values so a silent all-zero result
+    // is immediately obvious.
+    Mat src(16, 16, CV_8UC1);
+    for (int i = 0; i < src.rows; i++)
+        for (int j = 0; j < src.cols; j++)
+            src.at<uchar>(i, j) = (uchar)(i * src.cols + j);
+
+    const string fname = cv::tempfile(".ras");
+    ASSERT_TRUE(imwrite(fname, src)) << "imwrite failed for .ras";
+
+    Mat dst = imread(fname, IMREAD_GRAYSCALE);
+    ASSERT_FALSE(dst.empty()) << "imread returned empty Mat";
+    ASSERT_EQ(src.size(), dst.size());
+    ASSERT_EQ(src.type(), dst.type());
+
+    // Before the fix every pixel read back as 0 (black), so norm would equal
+    // the sum of all original values instead of 0.
+    EXPECT_EQ(0, cvtest::norm(src, dst, NORM_INF))
+        << "imread(IMREAD_GRAYSCALE) returned wrong pixel values for .ras — "
+           "expected exact roundtrip, got all-zero (black) image";
+
+    EXPECT_EQ(0, remove(fname.c_str()));
+}
+#endif
+
 TEST(Imgcodecs_Image, regression_9376)
 {
     String path = findDataFile("readwrite/regression_9376.bmp");