[update] 2d rotations to be locked so body rotation no longer changes from collision torque

Author: vmarcella

crates/lambda-rs-platform/src/physics/rapier2d.rs

@@ -1192,8 +1192,10 @@ impl PhysicsBackend2D {
 
 /// Builds a Rapier rigid body builder with `lambda-rs` invariants applied.
 ///
-/// Bodies created by this backend do not lock the 2D rotation axis. Dynamic
-/// bodies are expected to rotate in response to collisions.
+/// Bodies created by this backend lock 2D rotation so `RigidBody2D` rotation
+/// changes only through explicit `set_rotation()` calls. This preserves the
+/// current public 2D rigid-body contract, which excludes angular dynamics from
+/// simulation stepping.
 ///
 /// # Arguments
 /// - `body_type`: The integration mode for the rigid body.
@@ -1219,18 +1221,24 @@ fn build_rapier_rigid_body(
       return RigidBodyBuilder::fixed()
         .translation(translation)
         .rotation(rotation)
+        .angvel(0.0)
+        .lock_rotations()
         .linvel(linear_velocity);
     }
     RigidBodyType2D::Kinematic => {
       return RigidBodyBuilder::kinematic_velocity_based()
         .translation(translation)
         .rotation(rotation)
+        .angvel(0.0)
+        .lock_rotations()
         .linvel(linear_velocity);
     }
     RigidBodyType2D::Dynamic => {
       return RigidBodyBuilder::dynamic()
         .translation(translation)
         .rotation(rotation)
+        .angvel(0.0)
+        .lock_rotations()
         .linvel(linear_velocity)
         .additional_mass(additional_mass_kg);
     }

crates/lambda-rs/tests/physics_2d/colliders.rs

@@ -188,6 +188,49 @@ fn physics_2d_rectangle_collider_local_rotation_changes_motion() {
   return;
 }
 
+/// Ensures collision response does not rotate dynamic bodies during stepping.
+///
+/// This test uses an off-center collider attachment so ground contact would
+/// introduce torque if the backend allowed angular dynamics. The body's
+/// rotation MUST remain unchanged after collision resolution.
+#[test]
+fn physics_2d_collision_response_does_not_change_body_rotation() {
+  let mut world = PhysicsWorld2DBuilder::new().build().unwrap();
+
+  let ground = RigidBody2DBuilder::new(RigidBodyType::Static)
+    .with_position(0.0, -1.0)
+    .build(&mut world)
+    .unwrap();
+
+  Collider2DBuilder::rectangle(20.0, 0.5)
+    .build(&mut world, ground)
+    .unwrap();
+
+  let body = RigidBody2DBuilder::new(RigidBodyType::Dynamic)
+    .with_position(0.0, 4.0)
+    .build(&mut world)
+    .unwrap();
+
+  Collider2DBuilder::circle(0.5)
+    .with_offset(0.75, 0.0)
+    .build(&mut world, body)
+    .unwrap();
+
+  step_world(&mut world, DEFAULT_STEP_COUNT);
+
+  let rotation = body.rotation(&world).unwrap();
+  let position = body.position(&world).unwrap();
+
+  assert_in_range("rotation", rotation, -0.001, 0.001);
+  assert!(
+    position[1] > -0.25,
+    "body did not appear to collide with the ground: y={}",
+    position[1],
+  );
+
+  return;
+}
+
 /// Simulates a circle falling onto a platform with a rotated rectangle collider.
 ///
 /// # Arguments

docs/specs/physics/colliders-2d.md

@@ -3,8 +3,8 @@ title: "2D Colliders"
 document_id: "colliders-2d-2026-02-17"
 status: "living"
 created: "2026-02-17T23:08:44Z"
-last_updated: "2026-03-14T22:54:24Z"
-version: "0.1.3"
+last_updated: "2026-03-14T23:51:46Z"
+version: "0.1.4"
 engine_workspace_version: "2023.1.30"
 wgpu_version: "26.0.1"
 shader_backend_default: "naga"
@@ -276,11 +276,10 @@ Collision detection
 Collision response (normative)
 - Contact resolution MUST update dynamic-body motion such that penetrations are
   resolved and restitution and friction affect motion.
-- Collision response MAY change `RigidBody2D` rotation for dynamic bodies when
-  contacts introduce torque through collider shape or local offset.
-- The public API does not currently expose explicit angular-velocity controls,
-  but dynamic-body rotation observed through `RigidBody2D` state MUST remain
-  backend-consistent.
+- Collision response MUST NOT change `RigidBody2D` rotation during
+  `PhysicsWorld2D::step()`.
+- Collider shape and local offset MAY change linear contact response, but MUST
+  NOT introduce angular dynamics through public 2D rigid-body stepping.
 
 Material properties
 - `density` MUST affect mass properties for dynamic bodies when the body mass
@@ -441,5 +440,7 @@ Manual verification
 - 2026-02-17 0.1.0: Define 2D collider shapes and attachment APIs.
 - 2026-02-17 0.1.1: Specify defaults and mass recomputation rules.
 - 2026-02-17 0.1.2: Add local rotation, material struct, and polygon limits.
-- 2026-03-14 0.1.3: Align the specification with the implemented rotation,
-  testing, and demo behavior.
+- 2026-03-14 0.1.3: Align the specification with the implemented testing and
+  demo behavior.
+- 2026-03-14 0.1.4: Restore the no-angular-dynamics collision contract to
+  match `RigidBody2D`.