Add project setup: layout, pyproject, constants, Doxyfile, README

Set up the Blackjack project foundation for gateway MIL-001.

- pyproject.toml: Python >=3.13, src layout, no runtime dependencies,
  optional dev extras (ruff, mypy, pytest) and their configuration
- src/blackjack/: package with constants.py (deck, scoring thresholds,
  emoji card faces) and Doxygen-style docstrings
- tests/test_package.py: smoke tests for the package and its constants
- Doxyfile: reads src/, writes HTML to docs/doxygen/
- README.md: venv setup with pip upgrade, run, test, lint and Doxygen
  instructions
- .gitignore: ignore generated docs/doxygen/

The repository description and topics were set on the git host through
its API (no file change).

Closes #1
Closes #2
Closes #3
Closes #4
Closes #5
Closes #6

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Task: MIL-001#6

.gitignore

@@ -174,3 +174,6 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 
+
+# Generated Doxygen output
+docs/doxygen/