feat: JoltPhysicsSharp integration — physics simulation

- Engine.Physics project with JoltPhysicsSharp 2.21.0
- PhysicsWorld: PhysicsSystem init/update/dispose, body creation/removal,
  transform sync (Jolt body → ECS Transform)
- RigidBody ECS component: MotionType (Static/Kinematic/Dynamic),
  ShapeType (Box/Sphere), mass, friction, restitution, damping
- Factory methods: DynamicBox, DynamicSphere, StaticBox, StaticPlane
- Main loop integration: create bodies on first frame, step physics,
  sync transforms back to ECS
- Demo scene: 6 dynamic cubes + 3 dynamic spheres fall from height,
  static floor plane. Torus knot and textured cube remain static.
- Collision layers: NonMoving (floor), Moving (dynamic objects)
- JobSystemThreadPool for multi-threaded physics
- 66/66 tests passing, 0 warnings, 0 errors

CORTEX_ENGINE_ARCHITECTURE.md

@@ -723,7 +723,8 @@ In Release (NativeAOT), the MCP server and ASP.NET Core are excluded. The AI can
 - [ ] **Vulkan backend reintegration** — see §3.3 checklist. Compiles but untested.
     Kept architecturally compatible via `IWindow` / `IRenderContext` / `IRenderer`.
     Deferred because Raylib covers all current needs with far less complexity.
-- [ ] Physics (JoltPhysicsSharp)
+- [x] Physics (JoltPhysicsSharp 2.21.0) — Engine.Physics project, PhysicsWorld wrapper,
+  RigidBody ECS component, dynamic boxes/spheres with gravity, static floor, transform sync
 - [ ] AI hot-reload of C# scripts (Roslyn — conflicts with NativeAOT)
 - [ ] Semantic segmentation maps for MMLM vision input