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>

CLAUDE.md

@@ -21,13 +21,17 @@ renderer, camera, sprites), `Input`, `Audio`, `Assets` (runtime loading, no cont
 pipeline), `Assets.Generator` (Roslyn source generator for typed asset handles),
 `Atlases` (texture-atlas builder + runtime loader; CLI wrapper in `tools/MrGameEng.AtlasTool`),
 `Tilemaps` (code-built tile grids rendered through the batcher; `scene.UseTilemaps()`
-after `UseRenderer2D()`), `UI` (Myra integration: `scene.UseUI()` after `UseRenderer2D()`),
+after `UseRenderer2D()`), `Pathfinding` (grid A*/Dijkstra/BFS and flow fields over a
+game-implemented `IPathGrid`; Core-only, owns no world data), `Collisions` (`Collider`
+component, spatial hash rebuilt per tick, pairs/queries/raycast; `scene.UseCollisions()`
+after movement systems), `UI` (Myra integration: `scene.UseUI()` after `UseRenderer2D()`),
 `DevConsole` (in-game console capturing `Core.Log`; `scene.UseDevConsole()` last in OnLoad).
 Dependency rule: every module may depend only on `Core`; `Core` depends only on
 MonoGame and Friflo.Engine.ECS. `Assets.Generator` is a netstandard2.0 analyzer.
 Documented exceptions: Myra renders with its own SpriteBatch internally; `Atlases`
 depends on `Graphics` (Texture2DRegion) and `Assets` (loader registration);
-`Tilemaps` depends on `Graphics` (regions, layers, renderer).
+`Tilemaps` depends on `Graphics` (regions, layers, renderer);
+`Collisions` depends on `Graphics` (Transform2D, RectF).
 
 ## Commands