ui: show the "day being celebrated" label

Adds a real label under each hour's heading — "Monday in the 10th week
after Trinity", "The 1st Sunday of Advent" — computed from whichever
LiturgicalDay the hour actually resolved against, which can differ from
the nav date for Compline's evening anticipation.

calendar/day-label.ts combines two things: an ordinal week-within-season
label (pure date arithmetic on the season anchors from calendar/temporal.ts
and calendar/easter.ts — Trinity-counted, not Divinum Officium's own
Pentecost-counted convention; meant to become configurable later via the
same day->id indirection already used for the sanctoral calendar, not
hardcoded forever), and a feast name from calendar/commemorations.ts's
occurrence decision — shown alone if the feast displaces the day outright,
prefixed onto the temporal label if merely commemorated, or omitted
entirely if nothing's occurring.

tests/calendar/day-label.test.ts

@@ -0,0 +1,63 @@
+import { describe, expect, it } from 'vitest';
+import { resolveDay } from '../../src/calendar';
+import { getDayLabel } from '../../src/calendar/day-label';
+import type { LiturgicalDay } from '../../src/calendar/types';
+
+describe('getDayLabel — ordinal temporal label', () => {
+  it("labels an anchor's own partial week as \"Weekday after {Anchor}\"", () => {
+    // Trinity Sunday 2026 is May 31.
+    expect(getDayLabel(resolveDay('2026-06-01'))).toBe('Monday after Trinity Sunday');
+    expect(getDayLabel(resolveDay('2026-01-07'))).toBe('Wednesday after Epiphany'); // Epiphany 2026 is a Tuesday
+  });
+
+  it('names the anchor day itself, not "day after itself"', () => {
+    expect(getDayLabel(resolveDay('2026-05-31'))).toBe('Trinity Sunday');
+    expect(getDayLabel(resolveDay('2026-04-05'))).toBe('Easter');
+    expect(getDayLabel(resolveDay('2026-02-18'))).toBe('Ash Wednesday');
+  });
+
+  it('gives the correct week-after-Trinity ordinal', () => {
+    // Trinity Sunday 2026 is May 31; Jul 6 falls 5 full weeks after the
+    // first Sunday-after-Trinity (Jun 7).
+    expect(getDayLabel(resolveDay('2026-07-06'))).toBe('Monday in the 5th week after Trinity');
+  });
+
+  it("Advent's own anchor Sunday is already week 1, not a separate anchor-week case", () => {
+    expect(getDayLabel(resolveDay('2025-11-30'))).toBe('The 1st Sunday of Advent');
+    expect(getDayLabel(resolveDay('2025-12-01'))).toBe('Monday in the 1st week of Advent');
+  });
+
+  it('falls back to weekday + season name for seasons with no ordinal convention modeled', () => {
+    expect(getDayLabel(resolveDay('2026-05-15'))).toContain('in Ascensiontide');
+  });
+});
+
+describe('getDayLabel — feast name combination', () => {
+  const base: LiturgicalDay = {
+    date: '2026-06-15',
+    weekday: 'monday',
+    season: 'trinitytide',
+    temporalCategory: 'ordinary-feria',
+    occurring: [],
+  };
+
+  it('shows just the feast name when it wins outright', () => {
+    const day: LiturgicalDay = {
+      ...base,
+      occurring: [{ id: 'x', name: 'St. Ereden', rank: 'duplex', commemorated: false }],
+    };
+    expect(getDayLabel(day)).toBe('St. Ereden');
+  });
+
+  it('shows both, feast first, when the feast is merely commemorated', () => {
+    const day: LiturgicalDay = {
+      ...base,
+      occurring: [{ id: 'x', name: 'St. Ereden', rank: 'duplex-2-classis', commemorated: true }],
+    };
+    expect(getDayLabel(day)).toBe('St. Ereden — Monday in the 2nd week after Trinity');
+  });
+
+  it('shows just the temporal label when nothing is occurring at all', () => {
+    expect(getDayLabel(base)).toBe('Monday in the 2nd week after Trinity');
+  });
+});