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

src/dice/rulepacks/index.js

@@ -0,0 +1,6 @@
+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

@@ -0,0 +1,13 @@
+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 };
+  },
+};