ocaml: phase 6 Seq module (eager, list-backed) (+4 tests, 576 total)

Real OCaml's Seq.t is 'unit -> Cons of elt * Seq.t | Nil' — a lazy
thunk that lets you build infinite sequences. Ours is just a list,
which gives the right shape for everything in baseline programs that
don't rely on laziness (taking from infinite sequences would force
memory).

API: empty, cons, return, is_empty, iter, iteri, map, filter,
filter_map, fold_left, length, take, drop, append, to_list,
of_list, init, unfold.

unfold takes a step fn 'acc -> Option (elt * acc)' and threads
through until it returns None:

  Seq.fold_left (+) 0
    (Seq.unfold (fun n -> if n > 4 then None
                          else Some (n, n + 1))
                1)
  = 1 + 2 + 3 + 4 = 10

lib/ocaml/test.sh

@@ -1430,6 +1430,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5182)
 (eval "(ocaml-run-program \"module IntOrd = struct let compare a b = a - b end ;; module M = Map.Make (IntOrd) ;; let m = M.add 1 \\\"a\\\" (M.add 2 \\\"b\\\" M.empty) ;; M.cardinal m\")")
 
+;; ── Seq module (eager list-backed) ────────────────────────────
+(epoch 5190)
+(eval "(ocaml-run \"Seq.fold_left (+) 0 (Seq.map (fun x -> x * x) (Seq.of_list [1;2;3;4]))\")")
+(epoch 5191)
+(eval "(ocaml-run \"Seq.length (Seq.filter (fun x -> x mod 2 = 0) (Seq.of_list [1;2;3;4;5;6;7;8;9;10]))\")")
+(epoch 5192)
+(eval "(ocaml-run \"Seq.fold_left (+) 0 (Seq.init 5 (fun i -> i * 2))\")")
+(epoch 5193)
+(eval "(ocaml-run \"Seq.fold_left (+) 0 (Seq.unfold (fun n -> if n > 4 then None else Some (n, n + 1)) 1)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2274,6 +2284,12 @@ check 5180 "Set.Make dedupe sum"          '9'
 check 5181 "Set.Make mem"                 'true'
 check 5182 "Map.Make cardinal"            '2'
 
+# ── Seq module (eager list-backed) ──────────────────────────────
+check 5190 "Seq fold map squares"         '30'
+check 5191 "Seq filter evens"             '5'
+check 5192 "Seq init 5 i*2"               '20'
+check 5193 "Seq unfold 1..4 sum"          '10'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"