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>

lib/content/block-path.sx

@@ -0,0 +1,45 @@
+;; content-on-sx — locate a block in the tree (ancestor section path).
+;;
+;; The read-side companion to doc-find-deep (which returns the block) and the
+;; move/reparent ops (which relocate it): content/block-path returns the list of
+;; ancestor section ids, root-first, leading to a block id — i.e. where the
+;; block sits in the tree. A top-level block has an empty path; a block one
+;; section deep has a one-element path; a missing id returns nil (distinct from
+;; the empty-list path of a present top-level block). content/block-depth is the
+;; path length (0 = top level, -1 = absent). Useful for breadcrumbs and for
+;; scoping an edit to a block's enclosing section. Pure traversal; descends into
+;; any block carrying a children list, like the rest of the tree helpers.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  bp-in-blocks
+  (fn
+    (blocks id trail)
+    (if
+      (= (len blocks) 0)
+      nil
+      (let
+        ((b (first blocks)))
+        (if
+          (= (blk-id b) id)
+          trail
+          (let
+            ((ch (st-iv-get b "children")))
+            (let
+              ((found (if (list? ch) (bp-in-blocks ch id (append trail (list (blk-id b)))) nil)))
+              (if (= found nil) (bp-in-blocks (rest blocks) id trail) found))))))))
+
+;; ancestor section ids (root-first) for `id`, or nil if the block is absent.
+(define
+  content/block-path
+  (fn (doc id) (bp-in-blocks (doc-blocks doc) id (list))))
+
+;; depth of `id`: 0 at top level, n nested n sections deep, -1 if absent.
+(define
+  content/block-depth
+  (fn
+    (doc id)
+    (let
+      ((p (content/block-path doc id)))
+      (if (= p nil) -1 (len p)))))