content: tree-wide block transforms (transform.sx) + 12 tests (586/586)

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

lib/content/transform.sx

@@ -0,0 +1,52 @@
+;; content-on-sx — tree-wide block transforms.
+;;
+;; The write counterpart to query: apply a function to every matching block
+;; across the tree (descending into sections), returning a new document. For
+;; bulk edits — rewrite image srcs, bump heading levels, sanitise text. Tree
+;; detection/rebuild is inline (class + st-iv-get/set!) so this needs no
+;; section.sx. Immutable.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  xf-section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+(define
+  block-tree-transform
+  (fn
+    (blocks pred f)
+    (map
+      (fn
+        (b)
+        (let
+          ((nb (if (pred b) (f b) b)))
+          (if
+            (xf-section? nb)
+            (let
+              ((ch (st-iv-get nb "children")))
+              (if
+                (list? ch)
+                (st-iv-set! nb "children" (block-tree-transform ch pred f))
+                nb))
+            nb)))
+      blocks)))
+
+(define
+  content/map-blocks
+  (fn
+    (doc pred f)
+    (doc-with-blocks doc (block-tree-transform (doc-blocks doc) pred f))))
+
+(define
+  content/map-type
+  (fn
+    (doc type f)
+    (content/map-blocks doc (fn (b) (= (blk-type b) type)) f)))
+
+;; convenience: set a field on every block of a type.
+(define
+  content/set-field-on
+  (fn
+    (doc type field value)
+    (content/map-type doc type (fn (b) (blk-set b field value)))))