content: nested document outline (outline.sx) + 14 tests (635/635)

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

lib/content/outline.sx

@@ -0,0 +1,34 @@
+;; content-on-sx — nested document outline.
+;;
+;; Builds a hierarchical heading tree from content/headings: each node is
+;; {:id :text :level :children}, where a heading nests under the nearest
+;; preceding heading of a lower level. The structured companion to the flat TOC,
+;; for rendering nested navigation.
+;;
+;; Requires (loaded by harness): query.sx (content/headings).
+
+;; consume a prefix of `hs` forming nodes whose level > minlevel; return
+;; {:nodes ... :rest ...}.
+(define
+  ol-forest
+  (fn
+    (hs minlevel)
+    (if
+      (= (len hs) 0)
+      {:rest (list) :nodes (list)}
+      (let
+        ((h (first hs)))
+        (if
+          (<= (get h :level) minlevel)
+          {:rest hs :nodes (list)}
+          (let
+            ((sub (ol-forest (rest hs) (get h :level))))
+            (let
+              ((node {:id (get h :id) :text (get h :text) :children (get sub :nodes) :level (get h :level)}))
+              (let
+                ((more (ol-forest (get sub :rest) minlevel)))
+                {:rest (get more :rest) :nodes (cons node (get more :nodes))}))))))))
+
+(define
+  content/outline
+  (fn (doc) (get (ol-forest (content/headings doc) 0) :nodes)))