content: validation vets list items + table cells element-deep (787/787)

validate only checked that list items / table rows-headers ARE lists; a
non-string item or non-list/non-string-cell row passed yet crashes asText/
render/find-replace/search. Added ct-all-str?/ct-all-rows? + deepened list/
table branches (guarded against double-reporting). +9 validate tests.

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

lib/content/validate.sx

@@ -6,6 +6,11 @@
 ;; Tree detection is inline (class + st-iv-get) so this file needs no section.sx.
 ;; Dispatch on block type is a validation-boundary concern, not core behaviour.
 ;;
+;; Collection blocks are vetted element-deep: list items must all be strings and
+;; table rows must all be lists of strings — exactly what render/asText/
+;; find-replace/search assume — so malformed nested collections are caught at the
+;; boundary instead of crashing the render layer downstream.
+;;
 ;; Requires (loaded by harness): block.sx, doc.sx.
 
 (define ct-issue (fn (id kind detail) {:id id :detail detail :kind kind}))
@@ -36,6 +41,28 @@
 
 (define ct-uniq (fn (xs) (ct-uniq-loop xs (list))))
 
+;; every element a string? / every row a list of strings? (for collection blocks)
+(define
+  ct-all-str?
+  (fn
+    (xs)
+    (if
+      (= (len xs) 0)
+      true
+      (if (string? (first xs)) (ct-all-str? (rest xs)) false))))
+
+(define
+  ct-all-rows?
+  (fn
+    (rows)
+    (if
+      (= (len rows) 0)
+      true
+      (if
+        (if (list? (first rows)) (ct-all-str? (first rows)) false)
+        (ct-all-rows? (rest rows))
+        false))))
+
 ;; ── tree flatten (descends into CtSection children; guards malformed children) ──
 (define
   ct-section-block?
@@ -136,30 +163,43 @@
               "embed provider must be a string")))
         ((= t "divider") (list))
         ((= t "list")
-          (append
-            (ct-field-issue
-              id
-              (boolean? (blk-get b "ordered"))
-              "list ordered must be a boolean")
-            (ct-field-issue
-              id
-              (list? (blk-get b "items"))
-              "list items must be a list")))
+          (let
+            ((items (blk-get b "items")))
+            (append
+              (ct-field-issue
+                id
+                (boolean? (blk-get b "ordered"))
+                "list ordered must be a boolean")
+              (append
+                (ct-field-issue id (list? items) "list items must be a list")
+                (ct-field-issue
+                  id
+                  (if (list? items) (ct-all-str? items) true)
+                  "list items must all be strings")))))
         ((= t "section")
           (ct-field-issue
             id
             (list? (blk-get b "children"))
             "section children must be a list"))
         ((= t "table")
-          (append
-            (ct-field-issue
-              id
-              (list? (blk-get b "headers"))
-              "table headers must be a list")
-            (ct-field-issue
-              id
-              (list? (blk-get b "rows"))
-              "table rows must be a list")))
+          (let
+            ((headers (blk-get b "headers")) (rows (blk-get b "rows")))
+            (append
+              (append
+                (ct-field-issue
+                  id
+                  (list? headers)
+                  "table headers must be a list")
+                (ct-field-issue
+                  id
+                  (if (list? headers) (ct-all-str? headers) true)
+                  "table headers must all be strings"))
+              (append
+                (ct-field-issue id (list? rows) "table rows must be a list")
+                (ct-field-issue
+                  id
+                  (if (list? rows) (ct-all-rows? rows) true)
+                  "table rows must all be lists of strings")))))
         ((= t "callout")
           (append
             (ct-field-issue