refactor(fonts): reuse Dalamud IconFontFixedWidthHandle for FontAwesome

Drop the custom NewDelegateFontHandle that built our own FontAwesome
atlas slot and reuse Dalamud's UiBuilder.IconFontFixedWidthHandle
instead. One less delegate-build step in the ctor, and the handle is
host-managed so Dispose() leaves it alone.

The pre-cycle icon inventory verified that every site we push the
FontAwesome font for renders an icon that is present in the host's
fixed-width handle glyph range, so no rendering site changes.

HellionChat/FontManager.cs

@@ -13,15 +13,17 @@ namespace HellionChat;
 // AddFontWithFallback); a ctor-injected ILogger would not be reachable
 // from those scopes, so the class stays on Plugin.LogProxy.
 //
-// Hybrid handle model: Axis, AxisItalic and FontAwesome are tied to the
-// game's current font state and never change for the lifetime of the
-// plugin, so they are init-only. RegularFont and ItalicFont depend on
-// user-toggleable settings and get replaced live via RebuildDelegateFonts
-// when those settings change; they stay as mutable nullable fields.
+// Hybrid handle model: Axis and AxisItalic mirror the game's current
+// font state and are init-only. FontAwesome reuses Dalamud's UiBuilder
+// fixed-width icon handle and is likewise init-only. RegularFont and
+// ItalicFont depend on user-toggleable settings and get replaced live
+// via RebuildDelegateFonts when those settings change; they stay as
+// mutable nullable fields.
 //
-// All five handles register inside a single SuppressAutoRebuild block in
-// the ctor so the font atlas only rebuilds once for the whole plugin start
-// instead of once per handle.
+// The four atlas-owned handles register inside a single
+// SuppressAutoRebuild block so the font atlas only rebuilds once for the
+// whole plugin start instead of once per handle. FontAwesome lives
+// outside that accounting because the UiBuilder already owns it.
 public sealed class FontManager : IDisposable
 {
     private readonly IDalamudPluginInterface _pluginInterface;
@@ -83,13 +85,7 @@ public sealed class FontManager : IDisposable
                 }
             );
 
-            FontAwesome = atlas.NewDelegateFontHandle(e =>
-            {
-                e.OnPreBuild(tk =>
-                    tk.AddFontAwesomeIconFont(new SafeFontConfig { SizePx = GetFontSize() })
-                );
-                e.OnPostBuild(tk => tk.FitRatio(tk.Font));
-            });
+            FontAwesome = _pluginInterface.UiBuilder.IconFontFixedWidthHandle;
 
             RegularFont = BuildRegularFontHandle(atlas);
 
@@ -181,7 +177,8 @@ public sealed class FontManager : IDisposable
     {
         Axis.Dispose();
         AxisItalic.Dispose();
-        FontAwesome.Dispose();
+        // FontAwesome is shared with the UiBuilder; the host owns its
+        // lifetime, so the plugin must not dispose it.
         RegularFont?.Dispose();
         ItalicFont?.Dispose();
     }