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/tests/validate.sx

@@ -5,6 +5,7 @@
 (content-bootstrap-blocks!)
 (content-bootstrap-doc!)
 (content-bootstrap-section!)
+(content-bootstrap-table!)
 
 ;; ── a fully valid document ──
 (define
@@ -164,3 +165,62 @@
       (content/validate dup-tree)))
   1)
 (content-test "tree dup not valid" (content/valid? dup-tree) false)
+
+;; ── collection blocks vetted ELEMENT-DEEP (items/cells must be strings) ──
+;; A list whose items field is a list but holds a non-string would pass the old
+;; "is a list" check yet crash asText/render — now caught.
+(content-test
+  "list non-string item flagged"
+  (content/issue-kinds
+    (doc-append (doc-empty "d") (mk-list "l" true (list "a" 5))))
+  (list "field"))
+(content-test
+  "list all-string items valid"
+  (content/valid?
+    (doc-append (doc-empty "d") (mk-list "l" false (list "a" "b" "c"))))
+  true)
+(content-test
+  "list empty items valid"
+  (content/valid? (doc-append (doc-empty "d") (mk-list "l" true (list))))
+  true)
+;; a malformed-list block reports exactly one element issue (not the is-a-list one)
+(content-test
+  "list non-string item single issue"
+  (len
+    (content/validate
+      (doc-append
+        (doc-empty "d")
+        (mk-list "l" true (list 1 2)))))
+  1)
+
+(content-test
+  "valid table ok"
+  (content/valid?
+    (doc-append
+      (doc-empty "d")
+      (mk-table "t" (list "H1" "H2") (list (list "a" "b") (list "c" "d")))))
+  true)
+(content-test
+  "table empty rows valid"
+  (content/valid?
+    (doc-append (doc-empty "d") (mk-table "t" (list "H") (list))))
+  true)
+(content-test
+  "table non-list row flagged"
+  (content/issue-kinds
+    (doc-append (doc-empty "d") (mk-table "t" (list "H") (list "notarow"))))
+  (list "field"))
+(content-test
+  "table non-string cell flagged"
+  (content/issue-kinds
+    (doc-append
+      (doc-empty "d")
+      (mk-table "t" (list "H") (list (list "ok") (list 9)))))
+  (list "field"))
+(content-test
+  "table non-string header flagged"
+  (content/issue-kinds
+    (doc-append
+      (doc-empty "d")
+      (mk-table "t" (list "H" 2) (list (list "a" "b")))))
+  (list "field"))