content: document flatten (flatten.sx) + 10 tests (645/645)

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

lib/content/flatten.sx

@@ -0,0 +1,34 @@
+;; content-on-sx — document flatten.
+;;
+;; Un-nests a sectioned document into a flat block sequence: each section is
+;; replaced inline by its (recursively flattened) children, dropping the section
+;; wrapper. The inverse of content/wrap-section, for flat export targets.
+;; Immutable; inline tree handling (no section.sx dep).
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  flat-section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+(define
+  flat-blocks
+  (fn
+    (blocks)
+    (if
+      (= (len blocks) 0)
+      (list)
+      (let
+        ((b (first blocks)))
+        (append
+          (if
+            (flat-section? b)
+            (let
+              ((ch (st-iv-get b "children")))
+              (if (list? ch) (flat-blocks ch) (list)))
+            (list b))
+          (flat-blocks (rest blocks)))))))
+
+(define
+  content/flatten
+  (fn (doc) (doc-with-blocks doc (flat-blocks (doc-blocks doc)))))