ocaml: phase 6 Stack + Queue modules (+5 tests, 430 total)

Stack: ref-holding-list LIFO. push/pop/top/length/is_empty/clear.
Queue: two-list (front, back) amortised O(1) queue. push/pop/length/
is_empty/clear. Both in OCaml syntax in runtime.sx.

plans/ocaml-on-sx.md

@@ -267,6 +267,11 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [~] `Buffer`: `create`, `add_string`, `add_char`, `contents`, `length`,
       `clear`, `reset`. Backed by a ref holding a list of strings; reverse +
       `String.concat` on `contents`. Mostly-OCaml impl.
+- [~] `Stack`: `create`, `push`, `pop`, `top`, `is_empty`, `length`,
+      `clear`. Backed by a ref-holding-list (LIFO).
+- [~] `Queue`: `create`, `push`, `pop`, `is_empty`, `length`, `clear`.
+      Backed by a `(front, back)` tuple-of-lists pair (amortised O(1)
+      enqueue/dequeue via list reversal).
 - [~] `Sys`: `os_type` (`"SX"`), `word_size`, `max_array_length`,
       `max_string_length`, `executable_name`, `big_endian`, `unix`,
       `win32`, `cygwin`. Constants only; `argv`/`getenv_opt`/`command`
@@ -394,6 +399,11 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 6 — `Stack` and `Queue` modules in OCaml (+5 tests,
+  430 total). Stack: ref-holding-list LIFO with push/pop/top/length/
+  is_empty/clear. Queue: amortised O(1) two-list `(front, back)` queue
+  with push/pop/length/is_empty/clear. Both written entirely in OCaml
+  via lib/ocaml/runtime.sx.
 - 2026-05-08 Phase 5.1+1+2 — calc.ml baseline (11/11 pass) — a
   recursive-descent calculator parsing `(1 + 2) * 3 + 4` to 13. Two
   parser bugs fixed along the way: parse-let now handles inline