ocaml: phase 6 List.sort_uniq + List.find_map (+2 tests, 503 total)

sort_uniq:
  Sort with the user comparator, then walk the sorted list dropping
  any element equal to its predecessor. Output is sorted and unique.

  List.sort_uniq compare [3;1;2;1;3;2;4]  =  [1;2;3;4]

find_map:
  Walk until the user fn returns Some v; return that. If all None,
  return None.

  List.find_map (fun x -> if x > 5 then Some (x * 2) else None)
                [1;2;3;6;7]
  = Some 12

Both defined in OCaml syntax in runtime.sx — no host primitive
needed since they're pure list traversals over existing operations.

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — List.sort_uniq / List.find_map (+2 tests, 503
+  total). sort_uniq sorts then dedups consecutive equals. find_map
+  walks until the user fn returns `Some v` and returns it (or `None`
+  on empty/all-None). Closes two of the most-asked-for list ops; both
+  defined in OCaml syntax in runtime.sx.
 - 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 +