fix(comparison): stabilize line rendering, cut per-tick re-renders

Extract findSplitIndex; computeLineRenderModel now takes the split index
as a primitive. Line derives its model from `split`, so the $derived
short-circuits on value equality and skips recomputation on spring ticks
that don't move the split (previously every tick rebuilt the model and
re-rendered the line).

Lay the three regions out as inline boxes on a shared baseline. fontA and
fontB now align on the typographic baseline despite differing metrics,
and an always-present overflow:hidden strut pins the line-box baseline so
the line no longer jumps when a bulk run mounts/unmounts or the last
window char morphs to a font of different ascent.

src/entities/Font/lib/dualFontLayout/index.ts

@@ -5,5 +5,6 @@ export {
 } from './DualFontLayout/DualFontLayout';
 export {
     computeLineRenderModel,
+    findSplitIndex,
     type LineRenderModel,
 } from './computeLineRenderModel/computeLineRenderModel';