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

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _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 — Seq module (eager, list-backed) (+4 tests,
+  576 total). Real OCaml's Seq is lazy (a thunk producing
+  Cons / Nil); ours is just a list, which is adequate for most
+  baseline programs that don't rely on infinite sequences. 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. Lets us write
+  `Seq.fold_left (+) 0 (Seq.unfold (fun n -> if n > 4 then None else
+  Some (n, n + 1)) 1)` → 10.
 - 2026-05-09 Phase 5.1 — unique_set.ml baseline (Set.Make + IntOrd
   functor app, count uniques in [3;1;4;1;5;9;2;6;5;3;5;8;9;7;9] →
   9). First baseline that exercises the functor pipeline end to