ocaml: phase 2 let..and.. mutual recursion (+3 tests, 251 total)

Parser collects multiple bindings via 'and', emitting (:def-rec-mut
BINDINGS) for let-rec chains and (:def-mut BINDINGS) for non-rec.
Single bindings keep the existing (:def …) / (:def-rec …) shapes.

Eval (def-rec-mut): allocate placeholder cell per binding, build joint
env where each name forwards through its cell, then evaluate each rhs
against the joint env and fill the cells. Even/odd mutual-rec works.

lib/ocaml/test.sh

@@ -631,6 +631,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 832)
 (eval "(ocaml-run \"Result.is_error (Error \\\"oops\\\")\")")
 
+;; ── let ... and ... mutual recursion ──────────────────────────
+(epoch 850)
+(eval "(ocaml-run-program \"let rec even n = if n = 0 then true else odd (n - 1) and odd n = if n = 0 then false else even (n - 1);; even 10\")")
+(epoch 851)
+(eval "(ocaml-run-program \"let rec even n = if n = 0 then true else odd (n - 1) and odd n = if n = 0 then false else even (n - 1);; odd 7\")")
+(epoch 852)
+(eval "(ocaml-run-program \"let x = 1 and y = 2;; x + y\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1003,6 +1011,11 @@ check 830 "Result.map Ok"                '("Ok" 6)'
 check 831 "Result.is_ok"                 'true'
 check 832 "Result.is_error"              'true'
 
+# ── let ... and ... mutual recursion ─────────────────────────────
+check 850 "even 10 (mutual rec)"         'true'
+check 851 "odd 7 (mutual rec)"           'true'
+check 852 "let x = 1 and y = 2"          '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"