content: multi-doc index + tag filtering (index.sx) + 13 tests (710/710)

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

lib/content/index.sx

@@ -0,0 +1,51 @@
+;; content-on-sx — multi-document index.
+;;
+;; Projects a list of documents into summary cards (the blog index page), with
+;; tag filtering (category pages) and a tag cloud. Composes content/summary +
+;; doc metadata.
+;;
+;; Requires (loaded by harness): summary.sx (content/summary), meta.sx (doc-tags).
+
+(define
+  idx-in?
+  (fn
+    (x xs)
+    (cond
+      ((= (len xs) 0) false)
+      ((= (first xs) x) true)
+      (else (idx-in? x (rest xs))))))
+
+(define
+  idx-dedup
+  (fn
+    (xs seen)
+    (if
+      (= (len xs) 0)
+      (reverse seen)
+      (if
+        (idx-in? (first xs) seen)
+        (idx-dedup (rest xs) seen)
+        (idx-dedup (rest xs) (cons (first xs) seen))))))
+
+(define content/index (fn (docs) (map content/summary docs)))
+
+(define content/has-tag? (fn (doc tag) (idx-in? tag (doc-tags doc))))
+
+(define
+  content/index-by-tag
+  (fn
+    (docs tag)
+    (map content/summary (filter (fn (d) (content/has-tag? d tag)) docs))))
+
+(define
+  content/all-tags
+  (fn (docs) (idx-dedup (ct-flatmap-tags docs) (list))))
+
+(define
+  ct-flatmap-tags
+  (fn
+    (docs)
+    (if
+      (= (len docs) 0)
+      (list)
+      (append (doc-tags (first docs)) (ct-flatmap-tags (rest docs))))))