content: nested block trees (section.sx) + 25 tests (410/410)

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` → **385/385** (Phases 1–4 COMPLETE + 9 extensions: HTML/SX escaping, Markdown render+import, CRDT replication, validation, snapshot cache, doc metadata, plain-text render)
+`bash lib/content/conformance.sh` → **410/410** (Phases 1–4 COMPLETE + 10 extensions: HTML/SX escaping, Markdown render+import, CRDT replication, validation, snapshot cache, doc metadata, plain-text render, nested block trees)
 
 ## Ground rules
 
@@ -85,9 +85,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] snapshot cache over replay (`snapshot.sx`: cache-not-primary, transparent)
 - [x] document metadata (`meta.sx`: title/slug/tags + Ghost title plumbing)
 - [x] plain-text render + excerpt (`text.sx`: asText, content/excerpt)
+- [x] nested block trees (`section.sx`: CtSection container, recursive render, deep-find)
 
 ## Progress log
 
+- 2026-06-07 — Extension: nested block trees (`section.sx`). `CtSection` is a
+  block whose `children` ivar is a list of blocks (incl. nested sections →
+  arbitrary depth), turning the flat document into the ordered TREE from the
+  architecture sketch. Self-contained: it answers asHTML/asSx/asText/asMarkdown:
+  by folding children's renderings (pure polymorphic recursion — no changes to
+  block.sx/render.sx). `mk-section`, `section-children`, `section-append` (cow),
+  and tree traversal `doc-deep-find` / `doc-tree-ids` / `doc-tree-count` that
+  descend into sections. 25 tests; suite 410/410.
 - 2026-06-07 — Extension: plain-text render + excerpts (`text.sx`). Fourth
   boundary format via polymorphic `asText` (heading/text/code/quote→text,
   image→alt, embed/divider→"", list→", "-joined); the document joins non-empty