ocaml: phase 5 HM ctor inference for option/result (+7 tests, 351 total)

ocaml-hm-ctor-env registers None/Some : 'a -> 'a option, Ok/Error :
'a -> ('a, 'b) result. :con NAME instantiates the scheme; :pcon NAME
ARG-PATS walks arg patterns through the constructor's arrow type,
unifying each.

Pretty-printer renders 'Int option' and '(Int, 'b) result'.

Examples now infer:
  fun x -> Some x : 'a -> 'a option
  match Some 5 with | None -> 0 | Some n -> n : Int
  fun o -> match o with | None -> 0 | Some n -> n : Int option -> Int
  Ok 1 : (Int, 'b) result
  Error "oops" : ('a, String) result

User type-defs would extend the registry — pending.

plans/ocaml-on-sx.md

@@ -227,7 +227,10 @@ SX CEK evaluator (both JS and OCaml hosts)
       pattern matching, let-rec, modules.)_
 - [x] Type variables: `'a`, `'b`; unification with occur-check (kit).
 - [x] Let-polymorphism: generalise at let-bindings (kit `hm-generalize`).
-- [ ] ADT types: `type 'a option = None | Some of 'a`.
+- [~] ADT types: `option`/`result` ctors hardcoded in
+      `ocaml-hm-ctor-env`; `:con NAME` and `:pcon NAME …` look up the
+      scheme and instantiate. User type-defs would extend the registry.
+      Format: `Int option`, `(Int, 'b) result`.
 - [~] Function types `T1 -> T2` work; tuples (`'a * 'b`) and lists
       (`'a list`) supported. Records pending.
 - [ ] Type signatures: `val f : int -> int` — verify against inferred type.
@@ -365,6 +368,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — HM constructor inference for option/result (+7
+  tests, 351 total). `ocaml-hm-ctor-env` registers None/Some (`'a opt`),
+  Ok/Error (`('a, 'b) result`). `:con NAME` instantiates the scheme;
+  `:pcon NAME ARG-PATS` walks arg patterns through the constructor's
+  arrow type, unifying each. Pretty-printer renders `Int option` and
+  `(Int, 'b) result`. Examples:
+    `fun x -> Some x : 'a -> 'a option`
+    `fun o -> match o with | None -> 0 | Some n -> n : Int option -> Int`
 - 2026-05-08 Phase 5 — HM pattern-matching inference (+5 tests, 344
   total). `ocaml-infer-pat` covers wild, var, lit, cons, list, tuple,
   as. `ocaml-infer-match` unifies each clause's pattern type with the