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.

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