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.Pathfinding.Tests/TestGrid.cs

@@ -0,0 +1,23 @@
+using MrGameEng.Pathfinding;
+
+namespace MrGameEng.Pathfinding.Tests;
+
+/// <summary>Грид из строк: '#' — стена, '.' — клетка с ценой 1, '2'..'9' — клетка с этой ценой.</summary>
+public sealed class TestGrid : IPathGrid
+{
+    private readonly string[] _rows;
+
+    public TestGrid(params string[] rows) => _rows = rows;
+
+    public int Width => _rows[0].Length;
+
+    public int Height => _rows.Length;
+
+    public bool IsPassable(int x, int y) => _rows[y][x] != '#';
+
+    public float Cost(int x, int y)
+    {
+        var cell = _rows[y][x];
+        return cell is >= '2' and <= '9' ? cell - '0' : 1f;
+    }
+}