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.
Ilia Mashkov