host: block editor — edit the :body composition (composition roadmap step 6)

The post body is now editable as a composition. Model ops over the :body ref-seq (and the
ordered `contains` edges): host/blog-block-add! (create a card object is-a a card-type +
fields, contains edge, append a ref), -remove! (drop ref + edge), -move! (swap adjacent).
host/blog--block-editor renders a row per block — type + a content preview + ↑/↓/remove
controls + a "fields" link — plus an add-block form, injected into the edit page. Routes
POST /:slug/blocks/{add, :cslug/remove, :cslug/move} (guarded; SX-htmx sx-post + outerHTML
swap of #block-editor, redirect fallback for no-JS).

Cards-as-objects pays off: per-block FIELD editing is free — a card IS an object, so its
fields are edited via its own /<cslug>/edit page; the block editor only owns structure.
Guard fix: a card type is a SUBTYPE-OF card (not is-a), so the add validates ctype against
the down-closure of "card", not host/blog-is-a?. Verified via the warm server (162/164; the
2 fails are the pre-existing relate-picker pair). Deferred: Playwright live-swap check;
alt/each block insertion (the core editor handles the seq of refs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/tests/blog.sx

@@ -817,6 +817,39 @@
   (let ((html (dream-resp-body (host-bl-app (host-bl-req "/imp-x/")))))
     (list (contains? html "Heading One") (contains? html "Para text.") (contains? html "p.jpg")))
   (list true true true))
+;; -- block editor: structural edits to the post :body composition (step 6). --
+(host-bl-test "block-add! creates a card object + contains edge + appends a ref to the body"
+  (begin
+    (host/blog-put! "bdoc" "BDoc" "(article)" "published")
+    (host/blog--set-body! "bdoc" (quote (seq)))
+    (let ((c0 (host/blog-block-add! "bdoc" "card-text" {"text" "first"}))
+          (c1 (host/blog-block-add! "bdoc" "card-heading" {"level" "2" "text" "a head"})))
+      (list (host/blog-body-refs "bdoc")
+            (host/blog-is-a? c0 "card-text")
+            (contains? (host/blog-out "bdoc" "contains") c1))))
+  (list (list "bdoc__b0" "bdoc__b1") true true))
+(host-bl-test "block-move! reorders the body refs (and is a no-op at the ends)"
+  (begin
+    (host/blog-block-move! "bdoc" "bdoc__b1" "up")          ;; b1 before b0
+    (let ((after-up (host/blog-body-refs "bdoc")))
+      (host/blog-block-move! "bdoc" "bdoc__b1" "up")        ;; b1 already first -> no-op
+      (list after-up (host/blog-body-refs "bdoc"))))
+  (list (list "bdoc__b1" "bdoc__b0") (list "bdoc__b1" "bdoc__b0")))
+(host-bl-test "block-remove! drops the ref from the body + the contains edge"
+  (begin
+    (host/blog-block-remove! "bdoc" "bdoc__b1")
+    (list (host/blog-body-refs "bdoc") (contains? (host/blog-out "bdoc" "contains") "bdoc__b1")))
+  (list (list "bdoc__b0") false))
+(host-bl-test "the edit page shows the block editor (#block-editor + an add-block form)"
+  (let ((html (dream-resp-body (host-bl-wapp (host-bl-send "GET" "/bdoc/edit" "Bearer good" "" "")))))
+    (list (contains? html "block-editor") (contains? html "+ add block")))
+  (list true true))
+(host-bl-test "POST /bdoc/blocks/add (auth) adds a block -> body grows"
+  (begin
+    (host-bl-wapp (host-bl-send "POST" "/bdoc/blocks/add" "Bearer good"
+      "application/x-www-form-urlencoded" "ctype=card-text&text=added+block"))
+    (len (host/blog-body-refs "bdoc")))
+  2)
 (host-bl-test "a post with no schema'd type is vacuously valid"
   (host/blog-type-valid? "ppost" "(p \"anything\")") true)
 (host-bl-test "edit-submit rejects content violating the type schema (not saved)"