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/domain/index.ts

@@ -0,0 +1,10 @@
+export {
+    type ComparisonLine,
+    type ComparisonResult,
+    DualFontLayout,
+} from './DualFontLayout/DualFontLayout';
+export {
+    computeLineRenderModel,
+    findSplitIndex,
+    type LineRenderModel,
+} from './computeLineRenderModel/computeLineRenderModel';