fix: eliminate diagonal dither pattern from display driver

Root cause: Panel_ED2208's epd_quality mode applies _dither_row_rgb_pair
(diagonal bias pattern with dither=140) during _exec_transfer(). Since our
ImagePipeline already does proper Floyd-Steinberg dithering, the driver's
additional dithering was creating visible diagonal artifacts.

Fix: setEpdMode(epd_fastest) selects _dither_row_none (clean nearest-color
lookup, no spatial bias). On Panel_ED2208 this only affects the dither
function — NOT refresh quality or waveform.

Also: default dither_noise setting to OFF since error scatter was targeting
the wrong layer (it made gradient areas fuzzier without fixing the real
problem in the display driver).

Co-authored-by: Cursor <cursoragent@cursor.com>

test/test_native/test_photo_queue.cpp

@@ -101,6 +101,8 @@ void test_nearest_color_yellow();
 void test_dither_solid_black();
 void test_dither_solid_white();
 void test_dither_produces_valid_indices();
+void test_blue_noise_breaks_pattern_regularity();
+void test_blue_noise_dither_valid_indices();
 
 void setUp() {}
 void tearDown() {}
@@ -120,6 +122,8 @@ int main() {
     RUN_TEST(test_dither_solid_black);
     RUN_TEST(test_dither_solid_white);
     RUN_TEST(test_dither_produces_valid_indices);
+    RUN_TEST(test_blue_noise_breaks_pattern_regularity);
+    RUN_TEST(test_blue_noise_dither_valid_indices);
     UNITY_END();
     return 0;
 }