ocaml: phase 5.1 mat_mul.ml baseline (3x3 row-major matrix multiply, sum = 621)

Triple-nested for loop with row-major indexing:

  for i = 0 to n - 1 do
    for j = 0 to n - 1 do
      for k = 0 to n - 1 do
        c.(i * n + j) <- c.(i * n + j) + a.(i * n + k) * b.(k * n + j)
      done
    done
  done

For 3x3 matrices A=[[1..9]] and B=[[9..1]], the resulting C has sum
621. Tests deeply nested for loops on Array, Array.make + arr.(i) +
arr.(i) <- v + Array.fold_left.

50 baseline programs total — milestone.

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 — mat_mul.ml baseline (3x3 row-major matrix
+  multiply, sum of result = 621). Triple-nested for loop over
+  i / j / k with row-major indexing `c.(i * n + j) <- c.(i * n + j) +
+  a.(i * n + k) * b.(k * n + j)`. Tests deeply nested for loops on
+  Array, Array.make + arr.(i) + arr.(i) <- v + Array.fold_left, and
+  multi-arg let chains with intermediate Array bindings. 50 baseline
+  programs total — milestone.
 - 2026-05-09 Phase 5.1 — bsearch.ml baseline (binary search,
   position-sum 6 + 2 + (-1) = 7). Iterative bsearch on a sorted int
   array using two index refs `lo`/`hi` and a sentinel `found = -1`.