content: tree-CRDT orphan reparenting (no content loss on concurrent delete-section) + 4 tests (742/742)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/content/crdt-tree.sx

@@ -4,13 +4,14 @@
 ;; (the id of its containing section, "" = root) alongside its Logoot position.
 ;; Merge is still a join — it reuses crdt.sx's position/register/field merges and
 ;; adds parent (immutable, set once at insert). Materialisation rebuilds the
-;; ordered tree: root = elements with parent "", a section's children = elements
-;; whose parent is that section's id, each sorted by position. Commutative,
-;; associative, idempotent like the flat layer; concurrent inserts into the same
-;; or different parents converge deterministically.
+;; ordered tree: root = elements with parent "" (plus ORPHANS — elements whose
+;; parent is not a live section, e.g. after a concurrent delete-section +
+;; insert-child, so content is never silently lost); a section's children =
+;; elements whose parent is that section's id. Commutative/associative/idempotent
+;; like the flat layer.
 ;;
 ;; Requires (loaded by harness): crdt.sx (merge helpers + live/sort/materialise
-;; bits), block.sx, doc.sx, section.sx (mk-section).
+;; bits + crdt-member?), block.sx, doc.sx, section.sx (mk-section).
 
 (define ctt-merge-parent (fn (p1 p2) (if (= p1 nil) p2 p1)))
 
@@ -129,6 +130,34 @@
       (crdt-tree-apply-all (crdt-tree-apply state (first ops)) (rest ops)))))
 
 ;; ── materialise to a Phase-1 document (rebuild the ordered tree) ──
+(define
+  ctt-live-section-ids
+  (fn
+    (state)
+    (map
+      (fn (e) (get e :id))
+      (filter
+        (fn (e) (= (get e :type) "section"))
+        (crdt-live-elements state)))))
+
+;; an element belongs at root if its parent is "" or its parent is not a live
+;; section (orphan-reparenting: don't lose content when its section is deleted).
+(define
+  ctt-roots
+  (fn
+    (state)
+    (let
+      ((secids (ctt-live-section-ids state)))
+      (crdt-sort-by-pos
+        (filter
+          (fn
+            (e)
+            (if
+              (= (get e :parent) "")
+              true
+              (if (crdt-member? (get e :parent) secids) false true)))
+          (crdt-live-elements state))))))
+
 (define
   ctt-children
   (fn
@@ -157,8 +186,8 @@
     (doc-id state)
     (doc-new
       doc-id
-      (map (fn (e) (ctt-element->block state e)) (ctt-children state "")))))
+      (map (fn (e) (ctt-element->block state e)) (ctt-roots state)))))
 
 (define
   crdt-tree-order
-  (fn (state) (map (fn (e) (get e :id)) (ctt-children state ""))))
+  (fn (state) (map (fn (e) (get e :id)) (ctt-roots state))))