feat(physics): add voltex_physics crate with Collider and ContactPoint

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

crates/voltex_physics/src/contact.rs

@@ -0,0 +1,12 @@
+use voltex_ecs::Entity;
+use voltex_math::Vec3;
+
+#[derive(Debug, Clone, Copy)]
+pub struct ContactPoint {
+    pub entity_a: Entity,
+    pub entity_b: Entity,
+    pub normal: Vec3,
+    pub depth: f32,
+    pub point_on_a: Vec3,
+    pub point_on_b: Vec3,
+}