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.

lib/ocaml/tokenizer.sx

@@ -381,6 +381,20 @@
                         (slice src (+ start 1) pos)
                         start)
                       (step)))
+                  ;; Polymorphic variant tag: `Tag — emits a ctor token
+                  ;; identical to a nominal ctor. Runtime is dynamic, so
+                  ;; the distinction only matters for HM (deferred).
+                  ((and (= c "`")
+                        (< (+ pos 1) src-len)
+                        (ocaml-upper? (ocaml-peek 1)))
+                    (begin
+                      (advance! 1)
+                      (let ((ctor-start pos))
+                        (begin
+                          (when (and (< pos src-len) (ocaml-ident-char? (cur)))
+                            (begin (advance! 1) (read-ident ctor-start)))
+                          (push! "ctor" (slice src ctor-start pos) start)))
+                      (step)))
                   ((try-punct start) (step))
                   (else
                     (error