Implement the NATO converter with tests and Doxygen comments

Task: MIL-002#2
Refs #9
Refs #10
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Refs #13

tests/adapters/test_csv_alphabet.py

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+from pathlib import Path
+
+from nato_alphabet.adapters.csv_alphabet import CsvAlphabetSource, load_nato_dict
+
+EXPECTED_ENTRIES = 26
+
+
+def test_load_nato_dict_reads_temporary_csv(tmp_path: Path) -> None:
+    data_file = tmp_path / "alphabet.csv"
+    data_file.write_text("letter,code\na,Alfa\nB,Bravo\n", encoding="utf-8")
+
+    assert load_nato_dict(data_file) == {"A": "Alfa", "B": "Bravo"}
+
+
+def test_load_nato_dict_builds_all_entries_from_packaged_file() -> None:
+    alphabet = load_nato_dict()
+
+    assert len(alphabet) == EXPECTED_ENTRIES
+    assert alphabet["Z"] == "Zulu"
+
+
+def test_csv_alphabet_source_loads_given_file(tmp_path: Path) -> None:
+    data_file = tmp_path / "alphabet.csv"
+    data_file.write_text("letter,code\nA,Alfa\n", encoding="utf-8")
+
+    assert CsvAlphabetSource(data_file).load() == {"A": "Alfa"}