Add MrGameEng.Pathfinding and MrGameEng.Collisions modules

Pathfinding (depends on Core only): GridPathfinder with A* (octile/
Manhattan heuristic), Dijkstra and BFS over a game-implemented
IPathGrid; 4/8 connectivity, diagonals never cut corners. FlowField +
FlowFieldBuilder (multi-source Dijkstra) give crowds O(1) steering per
agent per frame. All buffers are grid-sized once and invalidated by a
generation stamp - repeated queries allocate nothing and clear nothing.

Collisions (Graphics exception: Transform2D, RectF): Collider component
(circle/AABB, offset, two-way layer masks), CollisionWorld - a uniform
spatial hash on flat arrays rebuilt from scratch each tick (O(n) for
movers, zero alloc after warm-up, deterministic pair order), pair
collection, QueryAabb and closest-hit Raycast. scene.UseCollisions()
registers CollisionSystem after movement systems.

30 new tests (string-map mazes, cost weighting, corner cutting, flow
descent; pair/mask/query/raycast). Sample gains a PathfindingScene
('path' console command): click to set the goal, 250 agents follow the
flow field, the A* path is highlighted, colliding agents flash red.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/MrGameEng.Collisions.Tests/CollisionWorldTests.cs

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+using Friflo.Engine.ECS;
+using Microsoft.Xna.Framework;
+using MrGameEng.Core;
+using MrGameEng.Graphics;
+using Xunit;
+
+namespace MrGameEng.Collisions.Tests;
+
+public class CollisionWorldTests
+{
+    private sealed class CollisionScene : Scene
+    {
+        public CollisionWorld World = null!;
+
+        protected override void OnLoad() => World = this.UseCollisions(cellSize: 32f);
+    }
+
+    private static (EngineContext Context, CollisionScene Scene) CreateScene()
+    {
+        var context = new EngineContext();
+        var scene = new CollisionScene();
+        context.Scenes.Switch(scene);
+        context.Scenes.Update(context.Clock); // применяет переключение и первый тик
+        return (context, scene);
+    }
+
+    private static Entity Spawn(Scene scene, Vector2 position, in Collider collider) =>
+        scene.Store.CreateEntity(Transform2D.At(position), collider);
+
+    private static void Tick(EngineContext context)
+    {
+        context.Clock.Advance(0.016f);
+        context.Scenes.Update(context.Clock);
+    }
+
+    [Fact]
+    public void OverlappingCircles_ProduceOnePair()
+    {
+        var (context, scene) = CreateScene();
+        var a = Spawn(scene, new Vector2(0f, 0f), Collider.Circle(10f));
+        var b = Spawn(scene, new Vector2(15f, 0f), Collider.Circle(10f));
+        Spawn(scene, new Vector2(100f, 100f), Collider.Circle(10f)); // далёкий — без пар
+
+        Tick(context);
+
+        var pair = Assert.Single(scene.World.Pairs.ToArray());
+        Assert.True((pair.A == a && pair.B == b) || (pair.A == b && pair.B == a));
+    }
+
+    [Fact]
+    public void SeparatedCircles_NoPairs()
+    {
+        var (context, scene) = CreateScene();
+        Spawn(scene, new Vector2(0f, 0f), Collider.Circle(5f));
+        Spawn(scene, new Vector2(11f, 0f), Collider.Circle(5f));
+
+        Tick(context);
+
+        Assert.Equal(0, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void CircleAndBox_Overlap_DetectedBothWays()
+    {
+        var (context, scene) = CreateScene();
+        Spawn(scene, new Vector2(0f, 0f), Collider.Box(20f, 20f));
+        Spawn(scene, new Vector2(14f, 0f), Collider.Circle(5f)); // касается правой грани
+
+        Tick(context);
+
+        Assert.Equal(1, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void CornerCircle_DoesNotTouchBox()
+    {
+        var (context, scene) = CreateScene();
+        Spawn(scene, new Vector2(0f, 0f), Collider.Box(20f, 20f));
+        // Угол бокса (10,10); круг r=5 в (16,16): расстояние до угла ~8.49 > 5.
+        Spawn(scene, new Vector2(16f, 16f), Collider.Circle(5f));
+
+        Tick(context);
+
+        Assert.Equal(0, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void LayerMasks_FilterPairs()
+    {
+        var (context, scene) = CreateScene();
+        var ghost = Collider.Circle(10f);
+        ghost.Layer = 0b10;
+        ghost.CollidesWith = 0b10; // призраки сталкиваются только с призраками
+
+        var wall = Collider.Circle(10f);
+        wall.Layer = 0b01;
+        wall.CollidesWith = 0b01;
+
+        Spawn(scene, new Vector2(0f, 0f), ghost);
+        Spawn(scene, new Vector2(5f, 0f), wall);
+
+        Tick(context);
+
+        Assert.Equal(0, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void MovingApart_PairDisappearsNextTick()
+    {
+        var (context, scene) = CreateScene();
+        Spawn(scene, new Vector2(0f, 0f), Collider.Circle(10f));
+        var mover = Spawn(scene, new Vector2(5f, 0f), Collider.Circle(10f));
+
+        Tick(context);
+        Assert.Equal(1, scene.World.Pairs.Length);
+
+        mover.GetComponent<Transform2D>().Position = new Vector2(100f, 0f);
+        Tick(context);
+        Assert.Equal(0, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void LargeCluster_AllTouchingPairsFound_OnceEach()
+    {
+        var (context, scene) = CreateScene();
+        // Цепочка из 10 кругов: касаются только соседи → ровно 9 пар.
+        for (var i = 0; i < 10; i++)
+        {
+            Spawn(scene, new Vector2(i * 18f, 0f), Collider.Circle(10f));
+        }
+
+        Tick(context);
+
+        Assert.Equal(9, scene.World.Pairs.Length);
+    }
+
+    [Fact]
+    public void QueryAabb_ReturnsOnlyEntitiesInArea()
+    {
+        var (context, scene) = CreateScene();
+        var inside = Spawn(scene, new Vector2(10f, 10f), Collider.Circle(5f));
+        Spawn(scene, new Vector2(200f, 200f), Collider.Circle(5f));
+
+        Tick(context);
+
+        Span<Entity> results = new Entity[8];
+        var found = scene.World.QueryAabb(new RectF(0f, 0f, 50f, 50f), results);
+
+        Assert.Equal(1, found);
+        Assert.Equal(inside, results[0]);
+    }
+
+    [Fact]
+    public void Raycast_HitsClosestCollider_AndRespectsMask()
+    {
+        var (context, scene) = CreateScene();
+        var near = Collider.Circle(5f);
+        near.Layer = 0b01;
+        var far = Collider.Circle(5f);
+        far.Layer = 0b10;
+
+        var nearEntity = Spawn(scene, new Vector2(30f, 0f), near);
+        var farEntity = Spawn(scene, new Vector2(60f, 0f), far);
+
+        Tick(context);
+
+        Assert.True(scene.World.Raycast(new Vector2(0f, 0f), new Vector2(100f, 0f), out var hit));
+        Assert.Equal(nearEntity, hit.Entity);
+        Assert.Equal(25f, hit.Point.X, 1);
+
+        Assert.True(scene.World.Raycast(new Vector2(0f, 0f), new Vector2(100f, 0f), out hit, mask: 0b10));
+        Assert.Equal(farEntity, hit.Entity);
+    }
+
+    [Fact]
+    public void Raycast_Miss_ReturnsFalse()
+    {
+        var (context, scene) = CreateScene();
+        Spawn(scene, new Vector2(0f, 50f), Collider.Box(10f, 10f));
+
+        Tick(context);
+
+        Assert.False(scene.World.Raycast(new Vector2(0f, 0f), new Vector2(100f, 0f), out _));
+    }
+}

tests/MrGameEng.Collisions.Tests/MrGameEng.Collisions.Tests.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit.v3" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\MrGameEng.Collisions\MrGameEng.Collisions.csproj" />
+  </ItemGroup>
+
+</Project>