ocaml: phase 3 polymorphic variants (+4 tests, 382 total)

Tokenizer recognises backtick followed by an upper ident, emitting a
ctor token identical to a nominal ctor. Parser and evaluator treat
polyvariants as ctors — same tagged-list runtime. So:

  `Red                   -> ("Red")
  `Some 42               -> ("Some" 42)
  match `Red with | `Red -> 1 | `Green -> 2 | `Blue -> 3
                          -> 1
  `Pair (1,2)            -> ("Pair" 1 2) (with tuple-arg flatten)

Proper row types in HM deferred.

plans/ocaml-on-sx.md

@@ -182,7 +182,10 @@ SX CEK evaluator (both JS and OCaml hosts)
       `(:exception-def NAME [ARG-TYPE-SRC])`. Runtime no-op since
       raise/match work on tagged ctor values. Built-ins:
       `Failure`/`Invalid_argument` via `failwith`/`invalid_arg`.
-- [ ] Polymorphic variants (surface syntax `\`Tag value`; runtime same tagged list).
+- [x] Polymorphic variants (surface syntax `` `Tag value ``; runtime same
+      tagged list as nominal ctors). Tokenizer recognises backtick + ctor;
+      parser/eval treat them identically to nominal ctors. Type system
+      handling deferred (proper row types).
 - [ ] Tests in `lib/ocaml/tests/adt.sx` — 40+ tests: ADTs, match, option/result.
 
 ### Phase 4 — Modules + functors
@@ -374,6 +377,12 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 3 — polymorphic variants `` `Tag `` (+4 tests, 382
+  total). Tokenizer recognises backtick followed by an upper ident,
+  tokenizing identically to nominal ctors. Parser and evaluator treat
+  them as ctors — same tagged-list runtime. Match patterns `` `Red ``
+  / `` `Pair (a, b) `` work without any extra wiring. Proper row
+  types in HM deferred.
 - 2026-05-08 Phase 6 — Float module: sqrt/sin/cos/pow/floor/ceil/round
   + pi constant (+6 tests, 378 total). Wraps host SX math primitives
   via `_float_*` builtins.