Add console interface: calculator loop, ASCII title and tests

MIL-003 Console interface.

- Add calculator() with recursive restart: y continues with the result,
  n starts a new calculation, q quits
- Show one ASCII title at start: a calculator with the lettering to
  its right
- Re-prompt on invalid numbers, operations and answers; restart on
  division by zero
- Add __main__.py for python -m calc and the calc console script
- Add interface tests with input() fed through monkeypatch

Closes #22
Closes #23
Closes #24
Closes #25

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4

tests/test_calculator.py

@@ -0,0 +1,79 @@
+"""!
+@file test_calculator.py
+@brief Tests for the interactive calculator loop.
+"""
+
+import pytest
+
+from calc.calculator import calculator, format_number, main
+
+
+def feed(monkeypatch: pytest.MonkeyPatch, answers: list[str]) -> None:
+    """!
+    @brief Make input() return the given answers in order.
+    @param monkeypatch The pytest monkeypatch fixture.
+    @param answers The lines the "user" types.
+    """
+    lines = iter(answers)
+    monkeypatch.setattr("builtins.input", lambda _prompt="": next(lines))
+
+
+def test_format_number_drops_trailing_zero() -> None:
+    assert format_number(5.0) == "5"
+    assert format_number(2.5) == "2.5"
+
+
+def test_calculator_adds_then_quits(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["2", "+", "3", "q"])
+    calculator()
+    assert "2 + 3 = 5" in capsys.readouterr().out
+
+
+def test_calculator_continues_with_previous_result(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["2", "+", "3", "y", "*", "4", "q"])
+    calculator()
+    assert "5 * 4 = 20" in capsys.readouterr().out
+
+
+def test_calculator_starts_new_calculation(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["2", "+", "3", "n", "10", "/", "4", "q"])
+    calculator()
+    assert "10 / 4 = 2.5" in capsys.readouterr().out
+
+
+def test_calculator_reprompts_on_invalid_input(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["abc", "1", "^", "+", "x", "1", "?", "q"])
+    calculator()
+    out = capsys.readouterr().out
+    assert "not a number" in out
+    assert "not a valid operation" in out
+    assert "Please answer" in out
+    assert "1 + 1 = 2" in out
+
+
+def test_calculator_restarts_after_division_by_zero(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["1", "/", "0", "6", "-", "1", "q"])
+    calculator()
+    out = capsys.readouterr().out
+    assert "cannot divide by zero" in out
+    assert "6 - 1 = 5" in out
+
+
+def test_main_shows_ascii_art(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    feed(monkeypatch, ["1", "+", "1", "q"])
+    main()
+    out = capsys.readouterr().out
+    assert "|_____________________|" in out
+    assert "Goodbye!" in out