LibGfx: Add IPC serialization for FontVariationSettings

The compositor IPC path needs variable-font axes to survive transport
with the font data that owns them. Add LibGfx-local specializations for
FontVariationSettings, compile the new implementation into LibGfx, and
expose the stored settings from Font so later font serializers can read
them without reaching into the class internals.

This is preparatory work required to add IPC between the main and
compositor threads.

Libraries/LibGfx/CMakeLists.txt

@@ -13,6 +13,7 @@ set(SOURCES
     Font/Font.cpp
     Font/FontDatabase.cpp
     Font/FontSupport.cpp
+    Font/FontVariationSettings.cpp
     Font/PathFontProvider.cpp
     Font/Typeface.cpp
     Font/TypefaceSkia.cpp

Libraries/LibGfx/Font/Font.h

@@ -75,6 +75,7 @@ public:
 
     Font const& bold_variant() const;
     hb_font_t* harfbuzz_font() const;
+    FontVariationSettings const& variation_settings() const { return m_font_variation_settings; }
     ShapeFeatures const& features() const { return m_shape_features; }
 
     struct ShapingCache {

Libraries/LibGfx/Font/FontVariationSettings.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/Font/FontVariationSettings.h>
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder& encoder, Gfx::FontVariationSettings const& settings)
+{
+    TRY(encoder.encode_size(settings.axes.size()));
+    for (auto const& axis : settings.axes) {
+        TRY(encoder.encode(axis.key.to_u32()));
+        TRY(encoder.encode(axis.value));
+    }
+    return {};
+}
+
+template<>
+ErrorOr<Gfx::FontVariationSettings> decode(Decoder& decoder)
+{
+    auto axis_count = TRY(decoder.decode_size());
+    Gfx::FontVariationSettings settings;
+    TRY(settings.axes.try_ensure_capacity(axis_count));
+    for (size_t i = 0; i < axis_count; ++i) {
+        auto tag = Gfx::FourCC::from_u32(TRY(decoder.decode<u32>()));
+        auto value = TRY(decoder.decode<float>());
+        TRY(settings.axes.try_set(tag, value));
+    }
+    return settings;
+}
+
+}

Libraries/LibGfx/Font/FontVariationSettings.h

@@ -10,6 +10,7 @@
 #include <AK/QuickSort.h>
 #include <AK/Vector.h>
 #include <LibGfx/FourCC.h>
+#include <LibIPC/Forward.h>
 
 namespace Gfx {
 
@@ -72,3 +73,13 @@ struct FontVariationSettings {
 };
 
 }
+
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder&, Gfx::FontVariationSettings const&);
+
+template<>
+ErrorOr<Gfx::FontVariationSettings> decode(Decoder&);
+
+}