Fix type inference in Physics sample for CI builds

Use explicit Entity type declarations instead of var to help compiler
type inference in CI environments where source generator timing may
cause tuple type inference to fail.

Author: claude

samples/KeenEyes.Sample.Physics/Program.cs

@@ -293,7 +293,7 @@ void RunRaycastingDemo()
     var targets = new List<(Entity entity, string name, Vector3 position)>();
 
     // Target 1: Directly ahead
-    var target1 = world.Spawn()
+    Entity target1 = world.Spawn()
         .With(new Transform3D(new Vector3(10, 2, 0), Quaternion.Identity, Vector3.One))
         .With(RigidBody.Static())
         .With(PhysicsShape.Sphere(1.0f))
@@ -302,7 +302,7 @@ void RunRaycastingDemo()
     targets.Add((target1, "Target A (ahead)", new Vector3(10, 2, 0)));
 
     // Target 2: To the right
-    var target2 = world.Spawn()
+    Entity target2 = world.Spawn()
         .With(new Transform3D(new Vector3(5, 2, 5), Quaternion.Identity, Vector3.One))
         .With(RigidBody.Static())
         .With(PhysicsShape.Box(1, 1, 1))
@@ -311,7 +311,7 @@ void RunRaycastingDemo()
     targets.Add((target2, "Target B (right)", new Vector3(5, 2, 5)));
 
     // Target 3: Above
-    var target3 = world.Spawn()
+    Entity target3 = world.Spawn()
         .With(new Transform3D(new Vector3(0, 8, 0), Quaternion.Identity, Vector3.One))
         .With(RigidBody.Static())
         .With(PhysicsShape.Sphere(0.5f))