refactor: remove QueryDescriptor from ISystem interface

Remove the mandatory Query property from the ISystem interface,
allowing systems to manage their own queries locally or via other
means. Update all implementing systems and tests to use local
query descriptors instead.

examples/TicTacToe/Systems/RenderSystem.cs

@@ -7,13 +7,6 @@ namespace TicTacToe;
 /// </summary>
 public class RenderSystem : ISystem
 {
-    public QueryDescriptor Query { get; }
-
-    public RenderSystem(World world)
-    {
-        Query = world.Query().With<Cell>().Build();
-    }
-
     public void Run(World world)
     {
         var state = world.ReadSingleton<GameState>();

examples/TicTacToe/Systems/WinCheckSystem.cs

@@ -8,13 +8,6 @@ namespace TicTacToe;
 /// </summary>
 public class WinCheckSystem : ISystem
 {
-    public QueryDescriptor Query { get; }
-
-    public WinCheckSystem(World world)
-    {
-        Query = world.Query().With<Cell>().With<Mark>().Build();
-    }
-
     public void Run(World world)
     {
         ref var state = ref world.GetSingleton<GameState>();
@@ -25,7 +18,8 @@ public class WinCheckSystem : ISystem
         // Build a 3×3 grid of marks using the iterator API.
         var grid = new Player[3, 3];
 
-        using var iter = world.Select<Cell, Mark>(Query);
+        var query = world.Query().With<Cell>().With<Mark>().Build();
+        using var iter = world.Select<Cell, Mark>(query);
         while (iter.MoveNext())
         {
             grid[iter.Current1.Row, iter.Current1.Col] = iter.Current2.Player;

src/OECS/ISystem.cs

@@ -2,15 +2,10 @@ namespace OECS;
 
 /// <summary>
 /// A system that runs on every tick.
-/// Systems declare a query and receive the world in their <see cref="Run"/> method.
+/// Systems receive the world in their <see cref="Run"/> method.
 /// </summary>
 public interface ISystem
 {
-    /// <summary>
-    /// The query that defines which entities this system operates on.
-    /// </summary>
-    QueryDescriptor Query { get; }
-
     /// <summary>
     /// Executes the system logic for this tick.
     /// </summary>

tests/OECS.Tests/CommandTests.cs

@@ -249,13 +249,10 @@ public class CommandTests
         private readonly World _world;
         private readonly SpawnCommand _command;
 
-        public QueryDescriptor Query { get; }
-
         public CommandEnqueueSystem(World world, SpawnCommand command)
         {
             _world = world;
             _command = command;
-            Query = world.Query().With<TestPosition>().Build();
         }
 
         public void Run(World world)
@@ -266,18 +263,17 @@ public class CommandTests
 
     private class EntityCountObserver : ISystem
     {
+        private readonly QueryDescriptor _query;
         public int SeenCount { get; private set; }
 
-        public QueryDescriptor Query { get; }
-
         public EntityCountObserver(World world)
         {
-            Query = world.Query().With<TestPosition>().Build();
+            _query = world.Query().With<TestPosition>().Build();
         }
 
         public void Run(World world)
         {
-            world.ForEach(Query, (Entity e, ref TestPosition pos) =>
+            world.ForEach(_query, (Entity e, ref TestPosition pos) =>
             {
                 SeenCount++;
             });

tests/OECS.Tests/QueryAndSystemTests.cs

@@ -175,11 +175,11 @@ public class SystemTests
 
     private class MovementSystem : ITickedSystem
     {
-        public QueryDescriptor Query { get; }
+        private readonly QueryDescriptor _query;
 
         public MovementSystem(World world)
         {
-            Query = world.Query().With<Position>().With<Velocity>().Build();
+            _query = world.Query().With<Position>().With<Velocity>().Build();
         }
 
         public void Run(World world) => Run(world, Tick.Logical());
@@ -187,7 +187,7 @@ public class SystemTests
         public void Run(World world, Tick tick)
         {
             float dt = tick.DeltaTime;
-            world.ForEach(Query, (Entity entity, ref Position pos, ref Velocity vel) =>
+            world.ForEach(_query, (Entity entity, ref Position pos, ref Velocity vel) =>
             {
                 pos.X += vel.X * dt;
                 pos.Y += vel.Y * dt;
@@ -232,13 +232,11 @@ public class SystemTests
 
     private class TestLogicalSystem : ITickedSystem
     {
-        public QueryDescriptor Query { get; }
         public bool WasCalled { get; private set; }
         public Tick ReceivedTick { get; private set; }
 
         public TestLogicalSystem(World world)
         {
-            Query = world.Query().With<Position>().Build();
         }
 
         public void Run(World world) => Run(world, Tick.Logical());
@@ -272,13 +270,10 @@ public class SystemTests
         private readonly int _id;
         private readonly List<int> _order;
 
-        public QueryDescriptor Query { get; }
-
         public OrderTrackingSystem(World world, int id, List<int> order)
         {
             _id = id;
             _order = order;
-            Query = world.Query().With<Position>().Build();
         }
 
         public void Run(World world)

tests/OECS.Tests/ReactivityTests.cs

@@ -268,12 +268,9 @@ public class ReactivityTests
     {
         private readonly World _world;
 
-        public QueryDescriptor Query { get; }
-
         public EntityCreatorSystem(World world)
         {
             _world = world;
-            Query = world.Query().With<Position>().Build();
         }
 
         public void Run(World world)

tests/OECS.Tests/SingletonTests.cs

@@ -148,12 +148,9 @@ public class SingletonTests
     {
         private readonly World _world;
 
-        public QueryDescriptor Query { get; }
-
         public TimeSystem(World world)
         {
             _world = world;
-            Query = world.Query().With<GameConfig>().Build();
         }
 
         public void Run(World world) => Run(world, Tick.Logical());