Add project configuration, skeleton and Doxyfile

Add pyproject.toml (Python >=3.13, no runtime dependencies, pytest as a
dev extra), the src/higher_lower package with an empty constants module
and an entry-point stub, a smoke test, and the Doxyfile for the Doxygen
comments in the source. Ignore the Doxygen output in .gitignore.

Verified in a fresh .venv: pip install -e ".[dev]", pytest (2 passed),
python -m higher_lower and doxygen Doxyfile without warnings.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Closes #1
Closes #2
Closes #3
Closes #4

tests/test_smoke.py

@@ -0,0 +1,16 @@
+"""@file test_smoke.py
+@brief Smoke test: the package and its entry point can be imported.
+"""
+
+import higher_lower
+from higher_lower import __main__ as entry_point
+
+
+def test_package_imports() -> None:
+    """@brief The package imports without side effects."""
+    assert higher_lower.__doc__
+
+
+def test_entry_point_is_callable() -> None:
+    """@brief The entry point exists and can be called."""
+    assert entry_point.main() is None