ocaml: phase 6 Hashtbl.iter / Hashtbl.fold (+2 tests, 494 total)

New host primitive _hashtbl_to_list returns the entries as a list of
OCaml tuples — ('tuple' k v) form, matching the AST representation
that the pattern-match VM (:ptuple) expects. Without that exact
shape, '(k, v) :: rest' patterns fail to match.

Hashtbl.iter / Hashtbl.fold in runtime walk that list with the user
fn. This closes a long-standing gap: previously Hashtbl was opaque
once values were written (we could only find_opt one key at a time).

  let t = Hashtbl.create 4 in
  Hashtbl.add t "a" 1; Hashtbl.add t "b" 2; Hashtbl.add t "c" 3;
  Hashtbl.fold (fun _ v acc -> acc + v) t 0   = 6

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 — Hashtbl.iter / Hashtbl.fold (+2 tests, 494
+  total). New host primitive `_hashtbl_to_list` returns the entries
+  as a list of OCaml tuples (`("tuple" k v)` form, matching the AST
+  representation that pattern matching expects). Hashtbl.iter / fold
+  in runtime walk that list with the user fn. Closes a long-standing
+  gap: previously Hashtbl was opaque after writing to it.
 - 2026-05-09 Phase 6 — Printf.sprintf with %d/%s/%f/%c/%b/%% (+4
   tests) and global `string_of_int`/`string_of_float`/`string_of_bool`
   (+1 test). 492 total. sprintf walks fmt char-by-char accumulating