content: nested-tree CvRDT (crdt-tree.sx) + 17 convergence tests (727/727)

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` → **710/710** (Phases 1–4 COMPLETE + ~33 extensions: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CRDT replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, HTML page wrapper + SEO page, doc composition + id-remap, portable data + wire serialization, block query + transforms + find/replace, TOC + anchored headings + outline, normalization)
+`bash lib/content/conformance.sh` → **727/727** (Phases 1–4 COMPLETE + ~34 extensions: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CvRDT flat + nested-tree + durable replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, HTML page wrapper + SEO page, doc composition + id-remap, portable data + wire serialization, block query + transforms + find/replace, TOC + anchored headings + outline, normalization)
 
 ## Ground rules
 
@@ -93,6 +93,7 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] media block (`media.sx`: CtMedia video/audio, renders html/sx/text/md, validated)
 - [x] list-card summary (`summary.sx`: content/summary — title/excerpt/words/reading/cover)
 - [x] multi-doc index (`index.sx`: content/index + index-by-tag + all-tags + has-tag?)
+- [x] nested-tree CvRDT (`crdt-tree.sx`: parent-aware, sections merge collaboratively)
 - [x] HTML page wrapper (`page.sx`: content/page, escaped title from metadata)
 - [x] SEO page (`page-full.sx`: content/page-full, lang + meta description from excerpt)
 - [x] document composition (`compose.sx`: concat/prepend/concat-all/wrap-section)
@@ -112,6 +113,15 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Extension: nested-tree CvRDT (`crdt-tree.sx`). Extends the flat
+  CvRDT to a TREE: each element carries a `parent` (containing section id, "" =
+  root) beside its Logoot pos; merge reuses crdt.sx's pos/register/field joins +
+  parent (immutable). Materialisation rebuilds the ordered tree (root + per-section
+  children sorted by pos, recursive). Sections now merge collaboratively; proven
+  commutative/associative/idempotent — same- and different-parent concurrent
+  inserts converge, nested sections, LWW, two-replica convergence. Reuses crdt.sx
+  + section.sx; flat crdt untouched (34/34). 17 tests; suite 727/727. This was
+  the flagged "research-grade" gap — done as a clean self-contained layer.
 - 2026-06-07 — Extension: multi-document index (`index.sx`). `content/index`
   projects a doc list into summary cards (blog index); `content/index-by-tag`
   filters by tag (category pages); `content/all-tags` is a deduped tag cloud;