Joints mate by default (Fusion's convention): origin B's z is turned against origin A's, so two picked faces meet face to face and B sits on A instead of inside it; Flip now means don't mate. File format 4 inverts older joints' flip so they keep their placement; as-built joints and rigid groups stay where they are

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/joints.rs

@@ -303,7 +303,7 @@ fn as_built(s: &mut Session, p: &Value) -> Result<Value> {
         b: JointOrigin { occurrence: ob, snap: fb },
         values: vec![0.0; n],
         limits: limits_param(s, p, kind, cmd)?,
-        flip: false,
+        flip: true, // as built: the origins coincide as they are, not mated
         offset: 0.0,
         angle: 0.0,
         suppressed: false,