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.

lib/ocaml/test.sh

@@ -1172,6 +1172,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4703)
 (eval "(ocaml-run \"((1 + 2) : int) * 3\")")
 
+;; ── Module body: def-mut / def-rec-mut ─────────────────────────
+(epoch 4800)
+(eval "(ocaml-run-program \"module M = struct let rec a n = if n = 0 then 0 else b (n - 1) and b n = if n = 0 then 1 else a (n - 1) end ;; M.a 5\")")
+(epoch 4801)
+(eval "(ocaml-run-program \"module M = struct let x = 1 and y = 2 end ;; M.x + M.y\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1857,6 +1863,10 @@ check 4701 "let f (x : int) : int"        '42'
 check 4702 "(5 : int)"                    '5'
 check 4703 "((1+2) : int) * 3"            '9'
 
+# ── Module body: def-mut / def-rec-mut ─────────────────────────
+check 4800 "module rec a/b mutual"        '1'
+check 4801 "module x and y"               '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"