content: Markdown doc export w/ frontmatter (md-doc.sx) + 12 tests (481/481)

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

lib/content/md-doc.sx

@@ -0,0 +1,63 @@
+;; content-on-sx — Markdown document export (frontmatter + body).
+;;
+;; content/markdown-doc emits a YAML-ish --- frontmatter block from the document
+;; metadata (title/slug/tags) followed by the Markdown body, completing the
+;; metadata round-trip with md/import (md/import ∘ content/markdown-doc keeps
+;; title/slug/tags). With no metadata it is just asMarkdown.
+;;
+;; Requires (loaded by harness): doc.sx, meta.sx (doc-title/slug/tags),
+;; markdown.sx (asMarkdown).
+
+(define mdd-nl (str "\n"))
+
+(define
+  mdd-join
+  (fn
+    (sep parts)
+    (cond
+      ((= (len parts) 0) "")
+      ((= (len parts) 1) (first parts))
+      (else (str (first parts) sep (mdd-join sep (rest parts)))))))
+
+(define
+  content/-fm-parts
+  (fn
+    (doc)
+    (append
+      (append
+        (if
+          (= (doc-title doc) nil)
+          (list)
+          (list (str "title: " (doc-title doc))))
+        (if
+          (= (doc-slug doc) nil)
+          (list)
+          (list (str "slug: " (doc-slug doc)))))
+      (let
+        ((tags (doc-tags doc)))
+        (if
+          (= (len tags) 0)
+          (list)
+          (list (str "tags: " (mdd-join ", " tags))))))))
+
+(define
+  content/-frontmatter
+  (fn
+    (doc)
+    (let
+      ((parts (content/-fm-parts doc)))
+      (if
+        (= (len parts) 0)
+        ""
+        (str "---" mdd-nl (mdd-join mdd-nl parts) mdd-nl "---")))))
+
+(define
+  content/markdown-doc
+  (fn
+    (doc)
+    (let
+      ((fm (content/-frontmatter doc)))
+      (if
+        (= fm "")
+        (asMarkdown doc)
+        (str fm mdd-nl mdd-nl (asMarkdown doc))))))