content: Markdown render mode (asMarkdown) + 20 tests (263/263)

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

plans/content-on-sx.md

@@ -19,7 +19,7 @@ injected adapter, not core.
 
 ## Status (rolling)
 
-`bash lib/content/conformance.sh` → **243/243** (Phases 1–4 COMPLETE + extensions: HTML + SX escaping)
+`bash lib/content/conformance.sh` → **263/263** (Phases 1–4 COMPLETE + extensions: HTML/SX escaping, Markdown render)
 
 ## Ground rules
 
@@ -78,9 +78,17 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 ## Extensions (post-roadmap)
 - [x] HTML escaping at the render boundary (`String>>htmlEscaped`: & < > ")
 - [x] asSx wire string-escaping (`String>>sxEscaped`: \ and " in SX literals)
+- [x] Markdown render mode (`asMarkdown:` / `content/render doc "md"`)
 
 ## Progress log
 
+- 2026-06-07 — Extension: Markdown render mode (`markdown.sx`). Third boundary
+  format alongside asHTML/asSx via the same polymorphic dispatch; blocks answer
+  `asMarkdown: nl` (boundary supplies the newline — this Smalltalk dialect has
+  no Character newline ctor). `content/render doc "md"`/`"markdown"`/`:md`,
+  `content/markdown`, `asMarkdown`. headings (`#`×level), fenced code, `> ` quote,
+  `![alt](src)`, `- `/`1. ` lists, `---`; doc joins blocks with a blank line. No
+  MD escaping yet. 20 tests; suite 263/263.
 - 2026-06-07 — Extension: asSx wire string-escaping. Added `String>>sxEscaped`
   (escapes `\`→`\\` then `"`→`\"`) and routed every `asSx` text/attr/list-item
   through it, so the SX wire format stays valid when content contains quotes or