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/runtime.sx

@@ -374,6 +374,43 @@
          else aux (i + 1) (acc ^ f (_string_get s i))
        in
        aux 0 \"\"
+     let iter f s =
+       let rec aux i =
+         if i >= _string_length s then ()
+         else (f (_string_get s i); aux (i + 1))
+       in
+       aux 0
+     let iteri f s =
+       let rec aux i =
+         if i >= _string_length s then ()
+         else (f i (_string_get s i); aux (i + 1))
+       in
+       aux 0
+     let fold_left f init s =
+       let rec aux i acc =
+         if i >= _string_length s then acc
+         else aux (i + 1) (f acc (_string_get s i))
+       in
+       aux 0 init
+     let fold_right f s init =
+       let rec aux i acc =
+         if i < 0 then acc
+         else aux (i - 1) (f (_string_get s i) acc)
+       in
+       aux (_string_length s - 1) init
+     let to_seq s =
+       let rec aux i =
+         if i >= _string_length s then []
+         else _string_get s i :: aux (i + 1)
+       in
+       aux 0
+     let of_seq xs =
+       let rec aux ys acc =
+         match ys with
+         | [] -> acc
+         | h :: t -> aux t (acc ^ h)
+       in
+       aux xs \"\"
    end ;;
 
    module Bytes = struct

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"

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 6 — String.iter / iteri / fold_left / fold_right /
+  to_seq / of_seq (+3 tests, 501 total). All implemented in OCaml
+  syntax inside the runtime stdlib; iter / iteri walk via index +
+  side-effecting `f`, fold_left / fold_right thread an accumulator,
+  to_seq returns a char list, of_seq concats a char list back to a
+  string. Round-trip: `String.of_seq (List.rev (String.to_seq
+  "hello"))` → "olleh".
 - 2026-05-09 Phase 5.1 — frequency.ml baseline + Format module alias
   (+2 tests, 498 total). frequency.ml builds a Hashtbl of char→count
   via `Hashtbl.find_opt` + `Hashtbl.replace` inside a `for` loop, then