ocaml: phase 5.1 brainfuck.ml baseline (subset interpreter)

Five '+++++.' groups, cumulative accumulator 5+10+15+20+25 = 75.

This is a brainfuck *subset* — only > < + - . (no [ ] looping). That's
intentional: the goal is to stress imperative idioms that the recently
added Array module + array indexing syntax + s.[i] make ergonomic, all
in one program.

Exercises:
  Array.make 256 0
  arr.(!ptr)
  arr.(!ptr) <- arr.(!ptr) + 1
  prog.[!pc]
  ref / ! / :=
  while + nested if/else if/else if for op dispatch

25 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 — brainfuck.ml baseline (subset interpreter,
+  five `+++++.` groups → cumulative 5+10+15+20+25 = 75). No loop
+  brackets — the interpreter only handles `> < + - .`, but that's
+  enough to exercise Array.make, arr.(i), arr.(i) <- v, prog.[!pc],
+  ref/!/:=, while loop with conditional update via nested if/else.
+  25 baseline programs total.
 - 2026-05-09 Phase 5.1 — sieve.ml baseline (Sieve of Eratosthenes,
   count of primes ≤ 50 = 15). Stresses Array.make + arr.(i) +
   arr.(i) <- v + nested for/while loops + `begin..end` block. 24