content: tree reparent — move-into section + promote (812/812)

insert/move were top-level only; a block could never move into/out of a
section. content/move-into (relocate to a section child at index, tree-wide)
+ content/promote (lift nested block to top level, subtree intact). Pure
tree transforms like the rest of move.sx; cycle-safe (rejects moving a block
into its own descendant). +13 tests.

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

lib/content/tests/move.sx

@@ -1,7 +1,8 @@
-;; Extension — relative block reorder.
+;; Extension — relative block reorder + tree reparent.
 
 (st-bootstrap-classes!)
 (content/bootstrap!)
+(content-bootstrap-section!)
 
 (define
   d
@@ -61,3 +62,84 @@
   "render after move"
   (asHTML (content/move-after d "a" "c"))
   "<p>B</p><p>C</p><p>A</p>")
+
+;; ── reparent: move a top-level block INTO a section ──
+(define
+  nd
+  (doc-append
+    (doc-append (doc-empty "d") (mk-text "p" "P"))
+    (mk-section "s" (list (mk-text "x" "X")))))
+(content-test
+  "move-into: block leaves top level"
+  (doc-ids (content/move-into nd "p" "s" 1))
+  (list "s"))
+(content-test
+  "move-into: block lands in section at index"
+  (doc-tree-ids (content/move-into nd "p" "s" 1))
+  (list "s" "x" "p"))
+(content-test
+  "move-into at front of section"
+  (doc-tree-ids (content/move-into nd "p" "s" 0))
+  (list "s" "p" "x"))
+(content-test "move-into immutable" (doc-tree-ids nd) (list "p" "s" "x"))
+
+;; ── reparent: move a NESTED block to a different section ──
+(define
+  two
+  (doc-append
+    (doc-append (doc-empty "d") (mk-section "s1" (list (mk-text "n" "N"))))
+    (mk-section "s2" (list (mk-text "y" "Y")))))
+(content-test
+  "move-into across sections"
+  (doc-tree-ids (content/move-into two "n" "s2" 1))
+  (list "s1" "s2" "y" "n"))
+
+;; ── promote: nested block out to top level (appended last) ──
+(content-test
+  "promote nested to top level"
+  (doc-tree-ids (content/promote two "n"))
+  (list "s1" "s2" "y" "n"))
+(content-test
+  "promote leaves section empty shell"
+  (doc-ids (content/promote two "n"))
+  (list "s1" "s2" "n"))
+(content-test
+  "promote a whole section keeps its subtree"
+  (doc-tree-ids
+    (content/promote
+      (doc-append
+        (doc-empty "d")
+        (mk-section "o" (list (mk-section "i" (list (mk-text "z" "Z"))))))
+      "i"))
+  (list "o" "i" "z"))
+
+;; ── cycle guard: cannot move a section into its own descendant ──
+(define
+  nest
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "outer"
+      (list (mk-section "inner" (list (mk-text "t" "T")))))))
+(content-test
+  "move section into its own child is a no-op"
+  (doc-tree-ids (content/move-into nest "outer" "inner" 0))
+  (list "outer" "inner" "t"))
+(content-test
+  "move block into itself is a no-op"
+  (doc-tree-ids (content/move-into nest "inner" "inner" 0))
+  (list "outer" "inner" "t"))
+
+;; ── reparent no-ops on missing ids ──
+(content-test
+  "move-into missing block no-op"
+  (doc-tree-ids (content/move-into nd "zzz" "s" 0))
+  (list "p" "s" "x"))
+(content-test
+  "move-into missing section no-op"
+  (doc-tree-ids (content/move-into nd "p" "zzz" 0))
+  (list "p" "s" "x"))
+(content-test
+  "promote missing no-op"
+  (doc-tree-ids (content/promote nd "zzz"))
+  (list "p" "s" "x"))