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

src/higher_lower/constants.py

@@ -0,0 +1,6 @@
+"""@file constants.py
+@brief Constants shared by the game modules.
+
+Constants are added here as the game logic (MIL-002) and the game loop
+(MIL-003) need them.
+"""