ocaml: phase 4 def-mut / def-rec-mut inside modules (+2 tests, 475 total)

ocaml-eval-module now handles :def-mut and :def-rec-mut decls so
'module M = struct let rec a n = ... and b n = ... end' works. The
def-rec-mut version uses cell-based mutual recursion exactly as the
top-level version.

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 4 — `:def-mut` / `:def-rec-mut` inside module
+  bodies (+2 tests, 475 total). `ocaml-eval-module` now handles
+  multi-binding `let .. and ..` decls. `module M = struct let rec a n =
+  ... and b n = ... end` works.
 - 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