ocaml: phase 5 parse ctor arg types in user type-defs (+3 tests, 409 total)

ocaml-hm-parse-type-src recognises primitive type names (int/bool/
string/float/unit), tyvars 'a, and simple parametric T list / T option.
Replaces the previous int-by-default placeholder in
ocaml-hm-register-type-def!.

So 'type tag = TStr of string | TInt of int' correctly registers
TStr : string -> tag and TInt : int -> tag. Pattern-match on tag
gives proper field types in the body. Multi-arg / function types
still fall back to a fresh tv.

plans/ocaml-on-sx.md

@@ -236,8 +236,10 @@ SX CEK evaluator (both JS and OCaml hosts)
       `ocaml-hm-register-type-def!` registers user types from `:type-def`.
       `ocaml-type-of-program` threads decls through the env, registering
       types and binding `let` schemes. `:con NAME` / `:pcon NAME …`
-      instantiate from the registry. _(Caveat: ctor arg types currently
-      default to `int` — proper type parsing pending.)_
+      instantiate from the registry. Ctor arg types parsed via
+      `ocaml-hm-parse-type-src` — handles primitives (`int`/`bool`/
+      `string`/`float`/`unit`), tyvars `'a`, simple parametric `T list`/
+      `T option`. Multi-arg/complex types fall back to a fresh tv.
 - [~] 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.
@@ -377,6 +379,12 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — parse simple type sources in user type-defs
+  (+3 tests, 409 total). `ocaml-hm-parse-type-src` recognises
+  primitive type names, tyvars `'a`, and `T list`/`T option`-style
+  parametric types. Replaces the old "default to Int" placeholder so
+  `type t = TStr of string` correctly registers `TStr : string -> t`.
+  Multi-arg / function types still fall back to a fresh tv.
 - 2026-05-08 Phase 6 — String extensions: ends_with/contains/trim/
   split_on_char/replace_all/index_of (+6 tests, 406 total). Wraps host
   primitives via `_string_*` builtins.