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/easter.ts

@@ -1,8 +1,29 @@
 // Computus (the date of Easter, and everything the temporal cycle hangs off
 // of it — Septuagesima, Ash Wednesday, Ascension, Pentecost, Trinity Sunday).
-// Not implemented yet: unused until milestone 4 (Lauds/Vespers), which is the
-// first hour content that actually varies by season. Deliberately left empty
-// rather than half-built ahead of need — see the "content stores" section of
-// the plan for why calendar-rule logic is being kept swappable rather than
-// front-loaded.
-export {};
+//
+// This is the Anonymous Gregorian algorithm (a.k.a. Meeus/Jones/Butcher),
+// valid for the whole Gregorian era (1583 onward). Hand-rolled rather than
+// pulling in a dependency: it's ~15 lines of pure integer arithmetic with a
+// long history of independent verification (see calendar/easter.test.ts),
+// not something that benefits from a library's abstraction or maintenance
+// surface for an app with zero runtime dependencies otherwise.
+import type { CalendarDate } from './date-math';
+
+export function easterSunday(year: number): CalendarDate {
+  const a = year % 19;
+  const b = Math.floor(year / 100);
+  const c = year % 100;
+  const d = Math.floor(b / 4);
+  const e = b % 4;
+  const f = Math.floor((b + 8) / 25);
+  const g = Math.floor((b - f + 1) / 3);
+  const h = (19 * a + b - d - g + 15) % 30;
+  const i = Math.floor(c / 4);
+  const k = c % 4;
+  const l = (32 + 2 * e + 2 * i - h - k) % 7;
+  const m = Math.floor((a + 11 * h + 22 * l) / 451);
+  const n = h + l - 7 * m + 114;
+  const month = Math.floor(n / 31);
+  const day = (n % 31) + 1;
+  return { year, month, day };
+}