content: document composition (compose.sx) + 17 tests (502/502)

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

lib/content/compose.sx

@@ -0,0 +1,42 @@
+;; content-on-sx — document composition.
+;;
+;; Combine documents (header + body + footer, templates, partials) into a new
+;; document. The result keeps the FIRST document's id and metadata; blocks are
+;; concatenated. Immutable — inputs are untouched. Block-id collisions across
+;; combined docs are the caller's concern (content/validate flags duplicates).
+;;
+;; Requires (loaded by harness): doc.sx.
+
+(define
+  content/concat
+  (fn (a b) (doc-with-blocks a (append (doc-blocks a) (doc-blocks b)))))
+
+(define
+  content/prepend
+  (fn (a b) (doc-with-blocks a (append (doc-blocks b) (doc-blocks a)))))
+
+(define
+  content/-concat-fold
+  (fn
+    (acc more)
+    (if
+      (= (len more) 0)
+      acc
+      (content/-concat-fold (content/concat acc (first more)) (rest more)))))
+
+(define
+  content/concat-all
+  (fn
+    (docs)
+    (if
+      (= (len docs) 0)
+      (doc-empty "merged")
+      (content/-concat-fold (first docs) (rest docs)))))
+
+;; wrap a document's blocks inside a single section (collapse to a subtree).
+;; Requires section.sx (mk-section) when used.
+(define
+  content/wrap-section
+  (fn
+    (doc section-id)
+    (doc-with-blocks doc (list (mk-section section-id (doc-blocks doc))))))