ocaml: phase 5 HM let-mut / let-rec-mut (+3 tests, 442 total)

ocaml-infer-let-mut: each rhs inferred in parent env, generalized
sequentially before adding to body env.

ocaml-infer-let-rec-mut: pre-bind all names with fresh tvs; infer
each rhs against the joint env, unify each with its tv, then
generalize all and infer body.

Mutual recursion now type-checks:
  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)
  in even   : Int -> Bool

lib/ocaml/test.sh

@@ -1084,6 +1084,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3708)
 (eval "(ocaml-run \"Bytes.concat \\\"-\\\" [\\\"a\\\";\\\"b\\\";\\\"c\\\"]\")")
 
+;; ── HM let-mut / let-rec-mut ──────────────────────────────────
+(epoch 3800)
+(eval "(ocaml-type-of \"let x = 1 and y = 2 in x + y\")")
+(epoch 3801)
+(eval "(ocaml-type-of \"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) in even\")")
+(epoch 3802)
+(eval "(ocaml-type-of \"let f x = x + 1 and g x = x * 2 in f 1 + g 2\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1715,6 +1723,11 @@ check 3706 "Result.fold Ok"               '50'
 check 3707 "Bytes.length"                 '5'
 check 3708 "Bytes.concat"                 '"a-b-c"'
 
+# ── HM let-mut / let-rec-mut ───────────────────────────────────
+check 3800 "let-mut x+y : Int"            '"Int"'
+check 3801 "let-rec-mut even"             '"Int -> Bool"'
+check 3802 "let-mut f and g"              '"Int"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"