refactor(font): extract glyph-comparison logic into a domain segment

Move DualFontLayout and computeLineRenderModel from lib/ to a new
domain/ segment. This is the pure glyph-comparison algorithm — no
framework, no UI, no shared/model dependencies — so it belongs in
domain per the FSD+ ui -> model -> domain interior rule, shielded from
UI-kit and API churn.

Public API is unchanged: the slice index re-exports domain/, so
$entities/Font consumers (ComparisonView SliderArea, Line) are
unaffected.

src/entities/Font/index.ts

@@ -1,4 +1,5 @@
 export * from './api';
+export * from './domain';
 export * from './lib';
 export * from './model';
 export * from './ui';

src/entities/Font/lib/index.ts

@@ -1,5 +1,3 @@
-export * from './dualFontLayout';
-
 export { getFontUrl } from './getFontUrl/getFontUrl';
 
 // Mock data helpers for Storybook and testing