feat(Pairing): add comboKey natural-key derivation

src/entities/Pairing/domain/comboKey/comboKey.test.ts

@@ -0,0 +1,20 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { comboKey } from './comboKey';
+
+describe('comboKey', () => {
+    it('derives a key from the two font ids', () => {
+        expect(comboKey({ id: 'x', headerFontId: 'Inter', bodyFontId: 'Lora' })).toBe('Inter|Lora');
+    });
+    it('ignores the surrogate id (content not identity)', () => {
+        const a = comboKey({ id: 'a', headerFontId: 'Inter', bodyFontId: 'Lora' });
+        const b = comboKey({ id: 'b', headerFontId: 'Inter', bodyFontId: 'Lora' });
+        expect(a).toBe(b);
+    });
+    it('is order-sensitive on role', () => {
+        expect(comboKey({ id: 'x', headerFontId: 'Lora', bodyFontId: 'Inter' })).toBe('Lora|Inter');
+    });
+});

src/entities/Pairing/domain/comboKey/comboKey.ts

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+import type { Pairing } from '../types';
+
+/**
+ * Natural key describing a Pairing's current fonts (not its identity).
+ * Used for URL share-encoding and "is this combo already on the board" checks.
+ * Recomputed on swap; two cards may share a comboKey but never an id.
+ *
+ * @param pairing - The pairing whose fonts form the key (its `id` is ignored).
+ * @returns The `headerFontId|bodyFontId` key.
+ */
+export function comboKey(pairing: Pairing): string {
+    return `${pairing.headerFontId}|${pairing.bodyFontId}`;
+}