feat: Year-Zero rule-pack + rule-pack registry

src/dice/rulepacks/index.js

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+import { yearZero } from './year-zero.js';
+
+const packs = new Map([[yearZero.id, yearZero]]);
+
+export function getRulePack(id) { return packs.get(id) || null; }
+export function registerRulePack(pack) { packs.set(pack.id, pack); }

src/dice/rulepacks/year-zero.js

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+export const yearZero = {
+  id: 'year-zero',
+  label: 'Year-Zero',
+  interpret(results) {
+    let successes = 0, ones = 0;
+    for (const r of results) {
+      if (r.value >= 10) successes += 2;
+      else if (r.value >= 6) successes += 1;
+      if (r.value === 1) ones += 1;
+    }
+    return { successes, ones, canPush: true, isCritical: successes >= 2 };
+  },
+};

tests/dice/year-zero.test.js

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+import { describe, it, expect } from 'vitest';
+import { yearZero } from '../../src/dice/rulepacks/year-zero.js';
+import { getRulePack, registerRulePack } from '../../src/dice/rulepacks/index.js';
+
+describe('year-zero rule-pack', () => {
+  it('counts 6-9 as one success and 10+ as two', () => {
+    const v = yearZero.interpret([{ sides: 8, value: 8 }, { sides: 8, value: 4 }, { sides: 12, value: 11 }]);
+    expect(v.successes).toBe(3); // 8 -> 1, 4 -> 0, 11 -> 2
+    expect(v.isCritical).toBe(true);
+  });
+
+  it('reports ones for stress on a push', () => {
+    const v = yearZero.interpret([{ sides: 6, value: 1 }, { sides: 6, value: 1 }, { sides: 6, value: 6 }]);
+    expect(v.ones).toBe(2);
+    expect(v.successes).toBe(1);
+    expect(v.canPush).toBe(true);
+  });
+
+  it('is resolvable from the registry by id', () => {
+    expect(getRulePack('year-zero')).toBe(yearZero);
+    expect(getRulePack('nope')).toBe(null);
+    const custom = { id: 'x', label: 'X', interpret: () => ({}) };
+    registerRulePack(custom);
+    expect(getRulePack('x')).toBe(custom);
+  });
+});