refactor: reorganize project structure and move examples

- Move examples (Blackjack, TicTacToe) to the root directory
- Convert example projects from Console applications to Libraries
- Add Directory.Build.props for shared build settings
- Update solution file and project references to reflect new paths

TicTacToe/Components/Cell.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace TicTacToe;
+
+/// <summary>
+/// Identifies a board position. One entity per cell.
+/// </summary>
+[MessagePackObject]
+public struct Cell
+{
+    [Key(0)] public int Row;
+    [Key(1)] public int Col;
+}

TicTacToe/Components/Mark.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace TicTacToe;
+
+/// <summary>
+/// A mark placed on a cell by a player.
+/// Only present on cells that have been claimed.
+/// </summary>
+[MessagePackObject]
+public struct Mark
+{
+    [Key(0)] public Player Player;
+}

TicTacToe/Components/Player.cs

@@ -0,0 +1,8 @@
+namespace TicTacToe;
+
+public enum Player : byte
+{
+    None = 0,
+    X = 1,
+    O = 2
+}