content: tree-wide content/find + has? (778/778)

Facade read-by-id was top-level only while content/edit's update/delete are
tree-wide — could not read back a nested block content/edit just modified.
Added generic ct-find-id (doc.sx) + doc-find-deep/doc-has-deep?; content/find
+ has? now descend into sections. content/find-top/has-top? keep top-level
lookup. Audit: remaining doc-find/ct-index-of callers are positional
insert/move (top-level by design). +6 api tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/content/doc.sx

@@ -5,9 +5,10 @@
 ;; and returns a NEW document — the input is never mutated, so any version is the
 ;; head of an op stream (replay-friendly for persist + CRDT merge).
 ;;
-;; By-id ops (update/delete) are TREE-WIDE: they descend into any block carrying
-;; a `children` list (i.e. sections), since ids are unique across the tree. This
-;; keeps the persist op-log and content/edit correct for nested documents.
+;; By-id ops (update/delete) and by-id lookup (doc-find-deep/doc-has-deep?) are
+;; TREE-WIDE: they descend into any block carrying a `children` list (i.e.
+;; sections), since ids are unique across the tree. This keeps the persist
+;; op-log, content/edit and content/find correct for nested documents.
 ;; insert/move are positional and act at the top level.
 ;;
 ;; CtDoc also carries optional metadata (title/slug/tags) — see meta.sx.
@@ -113,6 +114,26 @@
               b))))
       blocks)))
 
+;; tree-wide find by id: first block matching id anywhere in the tree, or nil.
+;; Descends into any `children` list, mirroring ct-replace-id/ct-remove-id.
+(define
+  ct-find-id
+  (fn
+    (blocks id)
+    (if
+      (= (len blocks) 0)
+      nil
+      (let
+        ((b (first blocks)))
+        (if
+          (= (blk-id b) id)
+          b
+          (let
+            ((ch (st-iv-get b "children")))
+            (let
+              ((nested (if (list? ch) (ct-find-id ch id) nil)))
+              (if (= nested nil) (ct-find-id (rest blocks) id) nested))))))))
+
 ;; ── query ──
 (define doc-index-of (fn (doc id) (ct-index-of (doc-blocks doc) id)))
 
@@ -128,6 +149,14 @@
   doc-has?
   (fn (doc id) (if (= (doc-index-of doc id) -1) false true)))
 
+;; tree-wide lookup by id — reads a nested block by the same id content/edit can
+;; update/delete (no section.sx dependency; uses the generic children descent).
+(define doc-find-deep (fn (doc id) (ct-find-id (doc-blocks doc) id)))
+
+(define
+  doc-has-deep?
+  (fn (doc id) (if (= (doc-find-deep doc id) nil) false true)))
+
 ;; ── structural edits (each returns a new document) ──
 (define doc-with-blocks (fn (doc blocks) (st-iv-set! doc "blocks" blocks)))
 
@@ -189,7 +218,7 @@
 ;; ── op constructors (data payload, reused by persist op log) ──
 (define op-insert (fn (block after) {:after after :op "insert" :block block}))
 
-(define op-update (fn (id field value) {:id id :field field :op "update" :value value}))
+(define op-update (fn (id field value) {:field field :id id :op "update" :value value}))
 
 (define op-move (fn (id index) {:id id :op "move" :index index}))