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/scoreboard.md

@@ -16,7 +16,7 @@ _Generated by `lib/content/conformance.sh`_
 | section | 25 | 0 | 25 |
 | compose | 17 | 0 | 17 |
 | tree-edit | 17 | 0 | 17 |
-| move | 11 | 0 | 11 |
+| move | 24 | 0 | 24 |
 | clone | 10 | 0 | 10 |
 | query | 20 | 0 | 20 |
 | toc | 8 | 0 | 8 |
@@ -46,4 +46,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **799** | **0** | **799** |
+| **Total** | **812** | **0** | **812** |