ocaml: phase 5.1 mutable_record.ml baseline (14/14 pass)

Counter-style record with two mutable fields. Validates the new
r.f <- v field mutation end-to-end through type decl + record literal
+ field access + field assignment + sequence operator.

  type counter = { mutable count : int; mutable last : int }
  let bump c = c.count <- c.count + 1 ; c.last <- c.count

After 5 bumps: count=5, last=5, sum=10.

lib/ocaml/baseline/expected.json

@@ -8,6 +8,7 @@
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "module_use.ml": 3,
+  "mutable_record.ml": 10,
   "option_match.ml": 5,
   "quicksort.ml": 44,
   "sum_squares.ml": 385,