Compose transition lifecycle and preserve authored attachments in both directions

crates/oa-core/src/transitions.rs

@@ -750,7 +750,7 @@ pub fn plan_transition(
                 );
                 continue;
             }
-            if !compatible_existing(project, *source, target, request.direction) {
+            if !counterpart_compatible(project, *source, target, request.direction) {
                 conflict(
                     &mut plan,
                     Some(*source),
@@ -1317,12 +1317,22 @@ fn compatible_kinds(source: Kind, target: Kind, direction: TransitionDirection)
     }
     inferred_kind(source, direction) == Some(target)
 }
-fn compatible_existing(
+/// Whether existing records have compatible layers, categories and kinds for
+/// an ordinary transition. This is candidate eligibility, not approval of a
+/// complete plan: properties, dependencies and structural conflicts still need
+/// the normal preview and apply validation. Manual traces are not restricted
+/// by this predicate.
+pub fn counterpart_compatible(
     p: &Project,
     source: Id,
     target: Id,
     direction: TransitionDirection,
 ) -> bool {
+    if p.layer_of(source) != Some(direction.source_layer())
+        || p.layer_of(target) != Some(direction.target_layer())
+    {
+        return false;
+    }
     match (p.elements.get(&source), p.elements.get(&target)) {
         (Some(a), Some(b)) => compatible_kinds(a.kind(), b.kind(), direction),
         _ => match (p.relations.get(&source), p.relations.get(&target)) {