Add Caesar cipher project with Doxygen docs

tests/test_cipher.py

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+from caesar_cipher import encrypt
+from caesar_cipher.env import load_env
+
+
+def test_encrypt_basic():
+    assert encrypt("hello", 5) == "mjqqt"
+
+
+def test_wraps_around_alphabet():
+    assert encrypt("xyz", 3) == "abc"
+    assert encrypt("abc", 29) == "def"
+
+
+def test_non_letters_and_case_preserved():
+    assert encrypt("Hi there, 42!", 1) == "Ij uifsf, 42!"
+
+
+def test_negative_shift():
+    assert encrypt("def", -3) == "abc"
+
+
+def test_load_env(tmp_path):
+    f = tmp_path / ".env"
+    f.write_text("# c\nA=1\nB='two'\n", encoding="utf-8")
+    assert load_env(f) == {"A": "1", "B": "two"}
+    assert load_env(tmp_path / "missing") == {}