feat(season): easter-offset + nth-weekday datumregler för marknads-högtider (MIKRO A)

packages/recommendation-engine/src/season.ts

@@ -54,7 +54,9 @@ export interface SeasonEventRuleInput {
   dateRule:
     | { kind: "fixed"; monthDay: string }
     | { kind: "range"; startMonthDay: string; endMonthDay: string }
-    | { kind: "computed"; algorithm: "midsummer" | "easter" | "advent" | "custom" };
+    | { kind: "computed"; algorithm: "midsummer" | "easter" | "advent" | "custom" }
+    | { kind: "easter-offset"; days: number }
+    | { kind: "nth-weekday"; month: number; weekday: number; n: number | "last" };
   leadDays: number;
 }
 
@@ -83,6 +85,19 @@ export function isEventActive(rule: SeasonEventRuleInput, today: Date): boolean
       if (end < start) end = Date.UTC(year + 1, (em ?? 1) - 1, ed ?? 1);
       return t >= start && t <= end;
     }
+    case "easter-offset": {
+      const target = new Date(easterSunday(year).getTime() + rule.dateRule.days * DAY);
+      return activeAround(target, 1);
+    }
+    case "nth-weekday": {
+      const target = nthWeekdayOfMonth(
+        year,
+        rule.dateRule.month,
+        rule.dateRule.weekday,
+        rule.dateRule.n,
+      );
+      return activeAround(target, 1);
+    }
     case "computed": {
       switch (rule.dateRule.algorithm) {
         case "midsummer":
@@ -102,3 +117,20 @@ export function isEventActive(rule: SeasonEventRuleInput, today: Date): boolean
 function strip(d: Date): Date {
   return new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate()));
 }
+
+/** N:te (eller sista) veckodagen i en månad. month 1–12, weekday 0=sön..6=lör. */
+function nthWeekdayOfMonth(
+  year: number,
+  month: number,
+  weekday: number,
+  n: number | "last",
+): Date {
+  if (n === "last") {
+    const last = new Date(Date.UTC(year, month, 0)); // sista dagen i månaden
+    const back = (last.getUTCDay() - weekday + 7) % 7;
+    return new Date(Date.UTC(year, month - 1, last.getUTCDate() - back));
+  }
+  const first = new Date(Date.UTC(year, month - 1, 1));
+  const fwd = (weekday - first.getUTCDay() + 7) % 7;
+  return new Date(Date.UTC(year, month - 1, 1 + fwd + (n - 1) * 7));
+}