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

pyproject.toml

@@ -0,0 +1,33 @@
+[build-system]
+requires = ["setuptools>=75"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "blackjack"
+version = "0.1.0"
+description = "Console Blackjack game against a computer dealer (Udemy 100 Days of Code, day 11)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = { file = "LICENSE" }
+dependencies = []
+
+[project.optional-dependencies]
+dev = [
+    "mypy",
+    "pytest",
+    "ruff",
+]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.ruff]
+line-length = 88
+src = ["src", "tests"]
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "B", "UP"]
+
+[tool.mypy]
+strict = true
+mypy_path = "src"