Add anti-aliased font rendering with dithering

Replace solid black rendering of anti-aliased font edges with period-2 dithering patterns. Full coverage pixels render as solid ink, while partial coverage pixels use checkerboard patterns (50% for dark grey, 25% for light grey) matching fillRectDither behavior. This prevents edge pixels from bolding to black on BW displays and produces proper anti-aliasing for 2-bit fonts.

lib/EpdFont/scripts/build-font-ids.sh

@@ -99,3 +99,12 @@ ruby -rdigest -e 'puts [
   "./notosans_8_regular.h",
 ].map{|f| Digest::SHA256.hexdigest(File.read(f)).to_i(16) }.sum % (2 ** 32) - (2 ** 31)'
 ))"
+
+echo ""
+echo "// Font ID 0 is reserved as the \"not found\" sentinel."
+echo "// Guard against any hash accidentally producing 0."
+for id in NOTOSERIF_12 NOTOSERIF_14 NOTOSERIF_16 NOTOSERIF_18 \
+          NOTOSANS_12 NOTOSANS_14 NOTOSANS_16 NOTOSANS_18 \
+          UI_10 UI_12 SMALL; do
+  echo "static_assert(${id}_FONT_ID != 0, \"Font ID collision with sentinel\");"
+done