feat: add Blackjack example

examples/Blackjack/Components/Card.cs

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+using MessagePack;
+
+namespace Blackjack;
+
+/// <summary>
+/// A playing card with a suit and rank.
+/// One entity per card.
+/// </summary>
+[MessagePackObject]
+public struct Card
+{
+    [Key(0)] public Suit Suit;
+    [Key(1)] public Rank Rank;
+}

examples/Blackjack/Components/DealerHand.cs

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+using MessagePack;
+
+namespace Blackjack;
+
+/// <summary>
+/// Tag component marking the dealer's hand entity.
+/// </summary>
+[MessagePackObject]
+public struct DealerHand { }

examples/Blackjack/Components/Deck.cs

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+using MessagePack;
+
+namespace Blackjack;
+
+/// <summary>
+/// Tag component marking the deck entity.
+/// </summary>
+[MessagePackObject]
+public struct Deck { }

examples/Blackjack/Components/Holds.cs

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+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Relationship from a hand entity to a card entity,
+/// representing that the hand holds this card.
+/// </summary>
+[MessagePackObject]
+public struct Holds : IRelationship
+{
+    [Key(0)] public Entity Source { get; set; }
+    [Key(1)] public Entity Target { get; set; }
+}

examples/Blackjack/Components/InDeck.cs

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+using MessagePack;
+using OECS;
+
+namespace Blackjack;
+
+/// <summary>
+/// Relationship from a card entity to the deck entity,
+/// representing that the card is in the deck.
+/// </summary>
+[MessagePackObject]
+public struct InDeck : IRelationship
+{
+    [Key(0)] public Entity Source { get; set; }
+    [Key(1)] public Entity Target { get; set; }
+}

examples/Blackjack/Components/PlayerHand.cs

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+using MessagePack;
+
+namespace Blackjack;
+
+/// <summary>
+/// Tag component marking the player's hand entity.
+/// </summary>
+[MessagePackObject]
+public struct PlayerHand { }

examples/Blackjack/Components/Rank.cs

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+namespace Blackjack;
+
+public enum Rank : byte
+{
+    Ace = 1,
+    Two = 2,
+    Three = 3,
+    Four = 4,
+    Five = 5,
+    Six = 6,
+    Seven = 7,
+    Eight = 8,
+    Nine = 9,
+    Ten = 10,
+    Jack = 11,
+    Queen = 12,
+    King = 13
+}

examples/Blackjack/Components/Suit.cs

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+namespace Blackjack;
+
+public enum Suit : byte
+{
+    Clubs = 0,
+    Diamonds = 1,
+    Hearts = 2,
+    Spades = 3
+}