ocaml: phase 6 String.iter/iteri/fold_left/fold_right/to_seq/of_seq (+3 tests, 501 total)

Six new String functions, all in OCaml syntax inside runtime.sx:

  iter      : index-walk with side-effecting f
  iteri     : iter with index
  fold_left : thread accumulator left-to-right
  fold_right: thread accumulator right-to-left
  to_seq    : return a char list (lazy in real OCaml; eager here)
  of_seq    : concat a char list back to a string

Round-trip:
  String.of_seq (List.rev (String.to_seq "hello"))   = "olleh"

Note: real OCaml's Seq is lazy. We return a plain list because the
existing stdlib already provides exhaustive list operations and we
don't yet have lazy sequences. If a baseline needs Seq.unfold or
similar, we'll graduate to a proper Seq module then.

lib/ocaml/test.sh

@@ -1240,6 +1240,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4971)
 (eval "(ocaml-run \"Format.asprintf \\\"%s=%d\\\" \\\"n\\\" 7\")")
 
+;; ── String.iter / fold / seq ─────────────────────────────────
+(epoch 4980)
+(eval "(ocaml-run \"let n = ref 0 in String.iter (fun c -> n := !n + Char.code c) \\\"abc\\\"; !n\")")
+(epoch 4981)
+(eval "(ocaml-run \"String.fold_left (fun acc c -> acc + Char.code c) 0 \\\"hi\\\"\")")
+(epoch 4982)
+(eval "(ocaml-run \"String.of_seq (List.rev (String.to_seq \\\"hello\\\"))\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1968,6 +1976,11 @@ check 4961 "lazy memoization counter=1"   '8401'
 check 4970 "Format.sprintf %d"            '"99"'
 check 4971 "Format.asprintf %s=%d"        '"n=7"'
 
+# ── String.iter / fold_left / seq ───────────────────────────────
+check 4980 "String.iter sum codes abc"    '294'
+check 4981 "String.fold_left sum hi"      '209'
+check 4982 "String.of_seq (rev to_seq)"   '"olleh"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"