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

src/blackjack/__init__.py

@@ -0,0 +1,6 @@
+"""!
+@file __init__.py
+@brief Console Blackjack game against a computer dealer.
+"""
+
+__version__ = "0.1.0"

src/blackjack/constants.py

@@ -0,0 +1,40 @@
+"""!
+@file constants.py
+@brief Constants for the Blackjack game; no magic numbers live in game code.
+"""
+
+## Value of an ace when it is counted as eleven.
+ACE_HIGH: int = 11
+
+## Value of an ace when it is counted as one.
+ACE_LOW: int = 1
+
+## The best possible score; a hand above it is bust.
+BLACKJACK_SCORE: int = 21
+
+## The dealer keeps drawing while its score is below this value.
+DEALER_STAND_SCORE: int = 17
+
+## Number of cards in the opening hand.
+OPENING_HAND_SIZE: int = 2
+
+## Score returned by the scoring function to mark a blackjack.
+BLACKJACK_MARKER: int = 0
+
+## Unlimited deck: cards are never removed. 11 is the ace, 10 covers 10/J/Q/K.
+CARDS: list[int] = [11, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10]
+
+## Emoji face for each card value.
+CARD_FACES: dict[int, str] = {
+    ACE_HIGH: "\U0001f170️",
+    ACE_LOW: "\U0001f170️",
+    2: "2️⃣",
+    3: "3️⃣",
+    4: "4️⃣",
+    5: "5️⃣",
+    6: "6️⃣",
+    7: "7️⃣",
+    8: "8️⃣",
+    9: "9️⃣",
+    10: "\U0001f51f",
+}