Gracefully handle 0 acceleration for walking targets

engine/physics/src/physics.rs

@@ -100,7 +100,6 @@ enum TransientAcceleration{
 		time:Time,
 	},
 	//walk target will never be reached
-	#[expect(dead_code)]
 	Unreachable{
 		acceleration:Planar64Vec3,
 	}
@@ -116,6 +115,10 @@ impl TransientAcceleration{
 	fn with_target_diff(target_diff:Planar64Vec3,accel:Planar64,time:Time)->Self{
 		if target_diff==vec3::ZERO{
 			TransientAcceleration::Reached
+		}else if accel==0{
+			TransientAcceleration::Unreachable{
+				acceleration:vec3::ZERO
+			}
 		}else{
 			//normal friction acceleration is clippedAcceleration.dot(normal)*friction
 			TransientAcceleration::Reachable{