ocaml: phase 5 HM let-rec + cons / append op types (+6 tests, 357 total)

ocaml-infer-let-rec pre-binds the function name to a fresh tv before
inferring rhs (which may recursively call the name), unifies the
inferred rhs type with the tv, generalizes, then infers body.

Builtin env types :: : 'a -> 'a list -> 'a list and @ : 'a list ->
'a list -> 'a list — needed because :op compiles to (:app (:app (:var
OP) L) R) and previously these var lookups failed.

Examples now infer:
  let rec fact n = if ... in fact : Int -> Int
  let rec len lst = ... in len    : 'a list -> Int
  let rec map f xs = ... in map   : ('a -> 'b) -> 'a list -> 'b list
  1 :: [2; 3]                      : Int list
  let rec sum lst = ... in sum [1;2;3] : Int

Scoreboard refreshed: 358/358 across 14 suites.

lib/ocaml/test.sh

@@ -872,6 +872,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1906)
 (eval "(ocaml-type-of \"fun o -> match o with | None -> 0 | Some n -> n\")")
 
+;; ── HM let-rec inference + cons / append ──────────────────────
+(epoch 2000)
+(eval "(ocaml-type-of \"let rec fact n = if n = 0 then 1 else n * fact (n - 1) in fact\")")
+(epoch 2001)
+(eval "(ocaml-type-of \"let rec len lst = match lst with | [] -> 0 | _ :: t -> 1 + len t in len\")")
+(epoch 2002)
+(eval "(ocaml-type-of \"let rec map f xs = match xs with | [] -> [] | h :: t -> f h :: map f t in map\")")
+(epoch 2003)
+(eval "(ocaml-type-of \"1 :: [2; 3]\")")
+(epoch 2004)
+(eval "(ocaml-type-of \"[1] @ [2; 3]\")")
+(epoch 2005)
+(eval "(ocaml-type-of \"let rec sum lst = match lst with | [] -> 0 | h :: t -> h + sum t in sum [1; 2; 3]\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1379,6 +1393,14 @@ check 1904 "fun x -> Some x"              ' option'
 check 1905 "match Some/None -> Int"       '"Int"'
 check 1906 "Int option -> Int"            '"Int option -> Int"'
 
+# ── HM let-rec + :: / @ ─────────────────────────────────────────
+check 2000 "type fact"                    '"Int -> Int"'
+check 2001 "type len"                     'list -> Int'
+check 2002 "type map"                     'list -> '
+check 2003 "type 1::list"                 '"Int list"'
+check 2004 "type [1] @ [2;3]"             '"Int list"'
+check 2005 "type sum"                     '"Int"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"