refactor(font): scaffold dualFontLayout module with shared types

src/entities/Font/lib/dualFontLayout/computeLineRenderModel/computeLineRenderModel.test.ts

@@ -0,0 +1,5 @@
+import { describe } from 'vitest';
+
+describe('computeLineRenderModel', () => {
+    // tests added in subsequent tasks
+});

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

@@ -0,0 +1,45 @@
+import type {
+    ComparisonChar,
+    ComparisonLine,
+} from '../DualFontLayout/DualFontLayout';
+
+/**
+ * Per-line render slice consumed by Line.svelte. The window is centered on the
+ * slider's split index and clamps at line boundaries.
+ */
+export interface LineRenderModel {
+    /**
+     * Chars before the window joined into a single string, rendered as one fontA text run.
+     */
+    leftText: string;
+    /**
+     * Window chars — each rendered as its own Character element with crossfade slots.
+     */
+    windowChars: Array<{
+        /**
+         * Stable key for Svelte keyed each — survives slider movement within the same line.
+         */
+        key: string;
+        /**
+         * Grapheme cluster to render.
+         */
+        char: string;
+        /**
+         * True once the slider has crossed this char's threshold.
+         */
+        isPast: boolean;
+    }>;
+    /**
+     * Chars after the window joined into a single string, rendered as one fontB text run.
+     */
+    rightText: string;
+}
+
+export function computeLineRenderModel(
+    line: ComparisonLine,
+    sliderPos: number,
+    containerWidth: number,
+    windowSize: number,
+): LineRenderModel {
+    throw new Error('not implemented');
+}