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/text.sx

@@ -0,0 +1,46 @@
+;; content-on-sx — plain-text render mode + excerpts.
+;;
+;; A fourth boundary format via polymorphic dispatch: blocks answer asText,
+;; stripping all markup. Useful for search indexing, meta descriptions and
+;; previews. The document joins non-empty child texts with a single space.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  content-bootstrap-text!
+  (fn
+    ()
+    (begin
+      (ct-def-method! "CtHeading" "asText" "asText ^ text")
+      (ct-def-method! "CtText" "asText" "asText ^ text")
+      (ct-def-method! "CtCode" "asText" "asText ^ text")
+      (ct-def-method! "CtQuote" "asText" "asText ^ text")
+      (ct-def-method! "CtImage" "asText" "asText ^ alt")
+      (ct-def-method! "CtEmbed" "asText" "asText ^ ''")
+      (ct-def-method! "CtDivider" "asText" "asText ^ ''")
+      (ct-def-method!
+        "CtList"
+        "asText"
+        "asText ^ (items inject: '' into: [:a :x | (a = '' ifTrue: [x] ifFalse: [a , ', ' , x])])")
+      (ct-def-method!
+        "CtDoc"
+        "asText"
+        "asText ^ (blocks inject: '' into: [:a :b | (b asText = '') ifTrue: [a] ifFalse: [(a = '' ifTrue: [b asText] ifFalse: [a , ' ' , b asText])]])")
+      true)))
+
+;; ── SX boundary ──
+(define asText (fn (node) (str (st-send node "asText" (list)))))
+(define content/text asText)
+(define block-text asText)
+
+;; excerpt: first n chars of the plain text, with an ellipsis if truncated.
+(define
+  content/excerpt
+  (fn
+    (doc n)
+    (let
+      ((t (asText doc)))
+      (if
+        (<= (string-length t) n)
+        t
+        (str (substring t 0 n) "…")))))