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/MrGameEng.Sample.csproj

@@ -13,6 +13,8 @@
     <ProjectReference Include="..\..\src\MrGameEng.Assets\MrGameEng.Assets.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Atlases\MrGameEng.Atlases.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Tilemaps\MrGameEng.Tilemaps.csproj" />
+    <ProjectReference Include="..\..\src\MrGameEng.Pathfinding\MrGameEng.Pathfinding.csproj" />
+    <ProjectReference Include="..\..\src\MrGameEng.Collisions\MrGameEng.Collisions.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.UI\MrGameEng.UI.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.DevConsole\MrGameEng.DevConsole.csproj" />
     <ProjectReference Include="..\..\src\MrGameEng.Assets.Generator\MrGameEng.Assets.Generator.csproj"