ocaml: phase 5.1 fizz_classifier.ml baseline (FizzBuzz with polymorphic variants, 1..30 weighted = 540)

Two functions:
  classify n   maps i to a polymorphic variant
                  FizzBuzz | Fizz | Buzz | Num of int
  score x      pattern-matches the variant to a weight
                  FizzBuzz=100, Fizz=10, Buzz=5, Num n=n

For i in 1..30:
  FizzBuzz at 15, 30:                  2 * 100 = 200
  Fizz at 3,6,9,12,18,21,24,27:        8 *  10 =  80
  Buzz at 5,10,20,25:                  4 *   5 =  20
  Num: rest (16 numbers)                       = 240
                                          total = 540

Exercises polymorphic-variant match (iter 87) including a
payload-bearing 'Num n' arm.

105 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — fizz_classifier.ml baseline (FizzBuzz with
+  polymorphic variants, 1..30 weighted score = 540). Two functions:
+  classify maps i → ` `FizzBuzz | `Fizz | `Buzz | `Num n``, and score
+  pattern-matches the variant to a weight (100/10/5/value). For
+  i=1..30: 200 + 80 + 20 + 240 = 540. Exercises polyvariant
+  match (iter 87) including a payload-bearing `` `Num n``. 105
+  baseline programs total.
 - 2026-05-09 Phase 5.1 — max_product3.ml baseline (max product of
   three from a list including negatives = 300). Sort, then compare
   product of three largest vs product of two smallest negatives and