content: plain-text render + excerpt (text.sx) + 20 tests (385/385)

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

lib/content/api.sx

@@ -49,8 +49,8 @@
     (if (content/op? ops) (doc-apply doc ops) (doc-apply-all doc ops))))
 
 ;; ── render boundary ──
-;; fmt is "html"/"sx"/"md" (or :html/:sx/:md — keywords evaluate to their name).
-;; "md" needs markdown.sx loaded.
+;; fmt is "html"/"sx"/"md"/"text" (or the matching keyword). "md" needs
+;; markdown.sx loaded; "text" needs text.sx loaded.
 (define
   content/render
   (fn
@@ -60,6 +60,7 @@
       ((= fmt "sx") (asSx doc))
       ((= fmt "md") (asMarkdown doc))
       ((= fmt "markdown") (asMarkdown doc))
+      ((= fmt "text") (asText doc))
       (else (error (str "unknown render format: " fmt))))))
 
 (define content/html asHTML)