content: fix ct-class-for-type for all block types (callout/media data round-trip) + 4 tests (731/731)

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

lib/content/block.sx

@@ -15,6 +15,10 @@
 ;;     CtEmbed (url provider)
 ;;     CtDivider
 ;;     CtList (ordered items)
+;; Plus self-contained blocks registered by their own files: CtSection,
+;; CtTable, CtCallout, CtMedia. ct-class-for-type maps every tag (so mk-block,
+;; content/from-data and CRDT materialise build them uniformly); the classes
+;; themselves are registered by content-bootstrap-section!/table!/callout!/media!.
 
 (define
   ct-def-method!
@@ -87,6 +91,10 @@
       ((= tag "embed") "CtEmbed")
       ((= tag "divider") "CtDivider")
       ((= tag "list") "CtList")
+      ((= tag "section") "CtSection")
+      ((= tag "table") "CtTable")
+      ((= tag "callout") "CtCallout")
+      ((= tag "media") "CtMedia")
       (else (error (str "unknown block type: " tag))))))
 
 ;; Generic constructor — wire tag + id + (name value) field pairs.