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/mutable_record.ml

@@ -0,0 +1,15 @@
+(* Baseline: mutable record fields via r.f <- v *)
+type counter = { mutable count : int; mutable last : int } ;;
+
+let bump c =
+  c.count <- c.count + 1 ;
+  c.last <- c.count
+;;
+
+let c = { count = 0; last = 0 } ;;
+bump c ;;
+bump c ;;
+bump c ;;
+bump c ;;
+bump c ;;
+c.count + c.last