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/test.sh

@@ -936,6 +936,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2405)
 (eval "(ocaml-run \"Float.ceil 3.2\")")
 
+;; ── Polymorphic variants ──────────────────────────────────────
+(epoch 2500)
+(eval "(ocaml-run \"\`Red\")")
+(epoch 2501)
+(eval "(ocaml-run \"\`Some 42\")")
+(epoch 2502)
+(eval "(ocaml-run \"match \`Red with | \`Red -> 1 | \`Green -> 2 | \`Blue -> 3\")")
+(epoch 2503)
+(eval "(ocaml-run \"match \`Pair (1, 2) with | \`Pair (a, b) -> a + b\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1480,6 +1490,12 @@ check 2403 "Float.pow 2 10"               '1024'
 check 2404 "Float.floor 3.7"              '3'
 check 2405 "Float.ceil 3.2"               '4'
 
+# ── Polymorphic variants ───────────────────────────────────────
+check 2500 "polyvar Red"                  '("Red")'
+check 2501 "polyvar Some 42"              '("Some" 42)'
+check 2502 "polyvar match"                '1'
+check 2503 "polyvar Pair (a,b)"           '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

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