feat(Pairing): add Pairing and Role types

src/entities/Pairing/domain/types/index.ts

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+export type {
+    Pairing,
+    Role,
+} from './pairing';

src/entities/Pairing/domain/types/pairing.ts

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+/**
+ * A slot within a Pairing that a font fills.
+ */
+export type Role = 'header' | 'body';
+
+/**
+ * The atomic unit of comparison: a header font + a body font.
+ * Carries a surrogate `id` (stable for the card's life, never tracks content)
+ * and the two font ids it pairs. Text and typography are global to the Board,
+ * not stored here.
+ */
+export interface Pairing {
+    /**
+     * Surrogate key generated at creation, stable for the card's life.
+     * Distinguishes duplicates with identical fonts. Focal/cycling key on this.
+     */
+    id: string;
+    /**
+     * Font entity id filling the header role.
+     */
+    headerFontId: string;
+    /**
+     * Font entity id filling the body role.
+     */
+    bodyFontId: string;
+}

src/entities/Pairing/model/types/index.ts

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+export type {
+    Pairing,
+    Role,
+} from './pairing';

src/entities/Pairing/model/types/pairing.ts

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+/**
+ * Re-export of the Pairing identity types. The source of truth lives in
+ * `domain/types` so the pure domain segment can reference them without importing
+ * `model` (FSD+ domain isolation: ui -> model -> domain, never back).
+ */
+export type {
+    Pairing,
+    Role,
+} from '../../domain/types';