refactor(font): scaffold dualFontLayout module with shared types

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

@@ -0,0 +1,87 @@
+/**
+ * Internal shape of pretext's PreparedTextWithSegments — only `segments` is in
+ * pretext's public TS type; the numeric arrays exist at runtime but are not in
+ * the published signature. Verified against node_modules/@chenglou/pretext/src/layout.ts.
+ */
+interface PretextInternals {
+    /**
+     * Per-segment text.
+     */
+    segments: string[];
+    /**
+     * Per-segment full width in pixels.
+     */
+    widths: number[];
+    /**
+     * Per-segment per-grapheme advance widths, or null when the segment is a single grapheme.
+     */
+    breakableFitAdvances: (number[] | null)[];
+    /**
+     * Per-segment line-end fit advance.
+     */
+    lineEndFitAdvances: number[];
+    /**
+     * Per-segment line-end paint advance.
+     */
+    lineEndPaintAdvances: number[];
+}
+
+/**
+ * Per-grapheme data computed during dual-font layout. Internal to the engine;
+ * consumed by computeLineRenderModel to derive the per-frame render model.
+ */
+export interface ComparisonChar {
+    /**
+     * Grapheme cluster (may be >1 code unit for emoji, combining marks).
+     */
+    char: string;
+    /**
+     * X offset from line start in fontA, pixels.
+     */
+    xA: number;
+    /**
+     * Advance width of this grapheme in fontA, pixels.
+     */
+    widthA: number;
+    /**
+     * X offset from line start in fontB, pixels.
+     */
+    xB: number;
+    /**
+     * Advance width of this grapheme in fontB, pixels.
+     */
+    widthB: number;
+}
+
+/**
+ * A single laid-out line. `chars` carries the per-grapheme data needed by
+ * computeLineRenderModel. Consumers should not iterate it directly.
+ */
+export interface ComparisonLine {
+    /**
+     * Full text of this line as returned by pretext.
+     */
+    text: string;
+    /**
+     * Rendered width in pixels — maximum across fontA and fontB.
+     */
+    width: number;
+    /**
+     * Per-grapheme metadata for both fonts.
+     */
+    chars: ComparisonChar[];
+}
+
+/**
+ * Aggregated output of a dual-font layout pass.
+ */
+export interface ComparisonResult {
+    /**
+     * Per-line grapheme data. Empty when input text is empty.
+     */
+    lines: ComparisonLine[];
+    /**
+     * Total height in pixels.
+     */
+    totalHeight: number;
+}

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

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

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

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+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');
+}

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

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+export type {
+    ComparisonLine,
+    ComparisonResult,
+} from './DualFontLayout/DualFontLayout';
+export {
+    computeLineRenderModel,
+    type LineRenderModel,
+} from './computeLineRenderModel/computeLineRenderModel';