ocaml: phase 5.1 bfs.ml baseline (20/20 pass)

Graph BFS using Queue + Hashtbl visited-set + List.assoc_opt + List.iter.
Returns 6 for a graph where A reaches B/C/D/E/F. Demonstrates 4 stdlib
modules (Queue, Hashtbl, List) cooperating in a real algorithm.

plans/ocaml-on-sx.md

@@ -407,6 +407,10 @@ _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.
+- 2026-05-08 Phase 5.1 — bfs.ml baseline (20/20 pass). Graph
+  breadth-first search using Queue + Hashtbl visited-set + List.assoc_opt
+  + List.iter. Returns the count of reachable nodes (6 for the demo
+  graph A→B→D→F, A→C→{D,E}, E→F).
 - 2026-05-08 Phase 1 — type annotations on let-bindings and parens
   expressions (+4 tests, 473 total). `let NAME [PARAMS] : T = expr`
   and `(expr : T)` parse and skip the type source. Runtime no-op