calendar: compute real Easter dates and temporal-cycle seasons

resolveDay() no longer hardcodes season: 'trinitytide'. Easter is
computed via the Anonymous Gregorian algorithm (verified against 16
known reference dates spanning 1583-2100); Advent/Christmastide/
Epiphanytide follow the Christmas-anchored rules (including Advent's
"Sunday nearest Nov 30"); Septuagesima through Trinitytide come from
easter-offsets.yml, reshaped from single named anchor points into real
ranges plus single-day overrides for Corpus Christi and Sacred Heart.

occurring feasts (calendar/feasts.ts) are still a stub — the sanctoral
calendar and Double-vs-not ranking are a separate, larger project.

src/calendar/date-math.ts

@@ -0,0 +1,27 @@
+// Small ISO-date ("YYYY-MM-DD") arithmetic helpers shared by easter.ts and
+// temporal.ts. Parsed/formatted as UTC midnight throughout, same convention
+// as weekday.ts, so results don't shift with the caller's local timezone.
+
+export interface CalendarDate {
+  year: number;
+  month: number; // 1-12
+  day: number;
+}
+
+export function toIsoDate(date: CalendarDate): string {
+  const mm = String(date.month).padStart(2, '0');
+  const dd = String(date.day).padStart(2, '0');
+  return `${date.year}-${mm}-${dd}`;
+}
+
+export function addDays(isoDate: string, days: number): string {
+  const d = new Date(`${isoDate}T00:00:00Z`);
+  d.setUTCDate(d.getUTCDate() + days);
+  return d.toISOString().slice(0, 10);
+}
+
+export function daysBetween(fromIsoDate: string, toIsoDate: string): number {
+  const from = new Date(`${fromIsoDate}T00:00:00Z`).getTime();
+  const to = new Date(`${toIsoDate}T00:00:00Z`).getTime();
+  return Math.round((to - from) / 86_400_000);
+}