feat(normalizer): Easter computus

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/import-normalizer/tests/test_dates.py

@@ -0,0 +1,9 @@
+import datetime
+import dates
+
+def test_easter_known_years():
+    # Anonymous Gregorian algorithm — verified against published tables
+    assert dates.easter(2024) == datetime.date(2024, 3, 31)
+    assert dates.easter(2000) == datetime.date(2000, 4, 23)
+    assert dates.easter(1922) == datetime.date(1922, 4, 16)
+    assert dates.easter(1888) == datetime.date(1888, 4, 1)