Implement the birthday wisher core (MIL-002)

Layered package (domain, application, infrastructure, presentation), CLI with
--dry-run and --today, SMTP with STARTTLS behind a port, 56 offline tests and
the Python code review RC-006.

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Refs #7
Refs #8
Refs #9
Refs #10
Refs #11
Refs #12

tests/unit/domain/test_person.py

@@ -0,0 +1,45 @@
+"""Unit tests for the Person entity."""
+
+from datetime import date
+
+from birthday_wisher.domain.person import Person
+
+
+def _person(month: int, day: int) -> Person:
+    return Person("Ann", "ann@example.com", 1990, month, day)
+
+
+def test_has_birthday_on_same_month_and_day() -> None:
+    assert _person(10, 17).has_birthday_on(date(2026, 10, 17))
+
+
+def test_has_birthday_on_other_day_is_false() -> None:
+    assert not _person(10, 17).has_birthday_on(date(2026, 10, 18))
+
+
+def test_has_birthday_on_same_day_other_month_is_false() -> None:
+    assert not _person(10, 17).has_birthday_on(date(2026, 11, 17))
+
+
+def test_has_birthday_on_ignores_year() -> None:
+    assert _person(10, 17).has_birthday_on(date(1999, 10, 17))
+
+
+def test_leap_day_birthday_is_29_feb_in_leap_year() -> None:
+    assert _person(2, 29).has_birthday_on(date(2024, 2, 29))
+
+
+def test_leap_day_birthday_is_not_28_feb_in_leap_year() -> None:
+    assert not _person(2, 29).has_birthday_on(date(2024, 2, 28))
+
+
+def test_leap_day_birthday_is_28_feb_in_non_leap_year() -> None:
+    assert _person(2, 29).has_birthday_on(date(2026, 2, 28))
+
+
+def test_leap_day_birthday_is_not_1_mar_in_non_leap_year() -> None:
+    assert not _person(2, 29).has_birthday_on(date(2026, 3, 1))
+
+
+def test_28_feb_birthday_is_28_feb_in_non_leap_year() -> None:
+    assert _person(2, 28).has_birthday_on(date(2026, 2, 28))