feat(physics): add Semi-implicit Euler integration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/voltex_physics/src/lib.rs

@@ -4,9 +4,11 @@ pub mod contact;
 pub mod narrow;
 pub mod collision;
 pub mod rigid_body;
+pub mod integrator;
 
 pub use bvh::BvhTree;
 pub use collider::Collider;
 pub use contact::ContactPoint;
 pub use collision::detect_collisions;
 pub use rigid_body::{RigidBody, PhysicsConfig};
+pub use integrator::integrate;