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>

samples/MrGameEng.Sample/Scenes/MainScene.cs

@@ -168,6 +168,13 @@ public sealed class MainScene : Scene
             }
         });
         console.Register("beep", "play the beep sound", (_, _) => audio.Play(beep));
+        console.Register("path", "switch to the pathfinding & collisions demo", (_, _) =>
+        {
+            if (!Context.Scenes.IsTransitioning)
+            {
+                Context.Scenes.Switch(new PathfindingScene(), Transition.Fade(0.6f));
+            }
+        });
         console.Register("atlas", "list texture atlas regions", (c, _) =>
         {
             c.WriteLine($"atlas '{atlas.Name}': {atlas.Pages.Count} page(s), {atlas.Regions.Count} region(s)");