Set up the project skeleton (MIL-001)

pyproject.toml (Python >=3.13, no runtime dependencies, pytest as dev extra),
Python .gitignore, .env.example, constants.py, assignment data files,
Doxyfile and a smoke test.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Refs #2
Refs #3
Refs #4
Refs #5

.gitignore

@@ -0,0 +1,29 @@
+# Secrets: personal use only, never committed
+.env
+
+# Python
+__pycache__/
+*.py[cod]
+*.egg-info/
+build/
+dist/
+
+# Virtual environments
+.venv/
+venv/
+
+# Test and tool caches
+.pytest_cache/
+.ruff_cache/
+.mypy_cache/
+.coverage
+htmlcov/
+
+# Generated documentation
+docs/doxygen/
+
+# Editors and OS
+.idea/
+.vscode/
+.DS_Store
+Thumbs.db