content: Markdown import adapter (md-import) + 24 tests (318/318)

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` → **294/294** (Phases 1–4 COMPLETE + extensions: HTML/SX escaping, Markdown, durable CRDT replication, validation)
+`bash lib/content/conformance.sh` → **318/318** (Phases 1–4 COMPLETE + extensions: HTML/SX escaping, Markdown render+import, durable CRDT replication, validation)
 
 ## Ground rules
 
@@ -81,9 +81,17 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] Markdown render mode (`asMarkdown:` / `content/render doc "md"`)
 - [x] durable CRDT replication (`crdt-store.sx`: ops on persist, replay + converge)
 - [x] document validation (`validate.sx`: ids, per-type fields, duplicate ids)
+- [x] Markdown import adapter (`md-import.sx`: text → blocks, round-trips export)
 
 ## Progress log
 
+- 2026-06-07 — Extension: Markdown import adapter (`md-import.sx`), inverse of
+  asMarkdown. Line-based parser: ATX headings, fenced code (```lang), blockquotes,
+  unordered/ordered lists (grouping consecutive items), thematic breaks,
+  paragraphs (consecutive plain lines joined with a space). Sequential ids
+  b0,b1…. `md/import` / `content/from-markdown` / `markdown-adapter` (import +
+  asMarkdown export). Round-trips canonical Markdown (import∘export == identity);
+  imported docs pass validation. 24 tests; suite 318/318.
 - 2026-06-07 — Extension: document validation (`validate.sx`). `content/validate`
   returns issue dicts `{:id :kind :detail}` (empty = valid); `content/valid?`
   and `content/issue-kinds` convenience. Checks block id (non-empty string),