feat(phase2): 타로 셔플·reference 순수 유틸 + 테스트

Fisher-Yates 셔플, 카드 픽 생성, 참고 블록/메타데이터 빌더 구현.
Task 4(interpret API)·Task 6(UI)에서 소비됨.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/tarot/reference.ts

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+import type { Pick } from './shuffle';
+
+export function buildReferenceBlock(picks: Pick[]): string {
+  return picks
+    .map((p, i) => {
+      const c = p.card;
+      const dir = p.reversed ? '역방향' : '정방향';
+      const kws = (p.reversed ? c.reversedKeywords : c.keywords).join(', ');
+      const meaning = p.reversed ? c.meaningReversed : c.meaningUpright;
+      const arcana = c.arcana === 'major' ? `Major (${c.id})` : `Minor (${c.suit})`;
+      return [
+        `## ${i + 1}. 위치: ${p.position} | 카드: ${c.nameEn} (${dir})`,
+        `- 아르카나: ${arcana}`,
+        `- 원소: ${c.element}`,
+        `- ${dir} 키워드: ${kws}`,
+        `- ${dir} 의미: ${meaning}`,
+      ].join('\n');
+    })
+    .join('\n\n');
+}
+
+export function buildContextMeta(picks: Pick[]) {
+  const major = picks.filter((p) => p.card.arcana === 'major').length;
+  const minor = picks.length - major;
+  const element_distribution: Record<string, number> = { air: 0, water: 0, fire: 0, earth: 0 };
+  for (const p of picks) element_distribution[p.card.element] += 1;
+  const orientation_flow = picks.map((p) => (p.reversed ? 'reversed' : 'upright')).join('→');
+  return { major_minor_ratio: `${major}:${minor}`, element_distribution, orientation_flow };
+}

lib/tarot/shuffle.ts

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+import type { TarotCard } from './cards';
+
+export type Pick = { card: TarotCard; position: string; reversed: boolean };
+
+export function fisherYates<T>(input: T[]): T[] {
+  const a = [...input];
+  for (let i = a.length - 1; i > 0; i -= 1) {
+    const j = Math.floor(Math.random() * (i + 1));
+    [a[i], a[j]] = [a[j], a[i]];
+  }
+  return a;
+}
+
+export function buildShuffle(deck: TarotCard[], size: number): (TarotCard & { reversed: boolean })[] {
+  return fisherYates(deck)
+    .slice(0, size)
+    .map((c) => ({ ...c, reversed: Math.random() < 0.5 }));
+}