content: content/block-path + block-depth — locate a block in the tree (825/825)

Read-side companion to doc-find-deep + reparent: ancestor-section chain
(root-first) for a block id, nil if absent (distinct from () top-level path);
block-depth is the path length. For breadcrumbs / scoping. New suite +13.
Probe this pass found no bugs: clone/remap tree-wide, all block types have
asMarkdown:.

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` → **812/812** (Phases 1–4 COMPLETE + ~34 extensions, hardened: 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)
+`bash lib/content/conformance.sh` → **825/825** (Phases 1–4 COMPLETE + ~34 extensions, hardened: 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
 
@@ -137,6 +137,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Feature: `content/block-path` + `content/block-depth`
+  (block-path.sx, new suite). The read-side companion to doc-find-deep (locate
+  the block) and move-into/promote (relocate it): returns the ancestor-section
+  id chain (root-first) for a block id — where it sits in the tree — or nil if
+  absent (distinct from the `()` path of a present top-level block). block-depth
+  is the path length (0 top-level, -1 absent). For breadcrumbs and scoping an
+  edit to a block's enclosing section; distinct from toc/outline (which work on
+  headings). Pure traversal. Also ran an adversarial probe this pass: confirmed
+  clone/remap-ids + prefix-ids are tree-wide, and all 12 block types + CtDoc have
+  asMarkdown: methods (no missing-render-method bug). +13 tests. 825/825 (43
+  suites).
+
 - 2026-06-07 — Feature: tree reparent in move.sx. Until now insert/move were
   positional and top-level only, so a block could never be moved *into* a section
   or *out* of one — a real gap for editing nested documents. Added