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.

plans/ocaml-on-sx.md

@@ -368,6 +368,18 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 5 — HM let-rec inference + `::`/`@` operator types
+  (+6 tests, 357 total). `ocaml-infer-let-rec` pre-binds the function
+  name to a fresh tv, infers rhs (which can recursively call name),
+  unifies inferred-rhs-type with the tv, generalizes, then infers
+  body. Builtin env now types `:: : 'a -> 'a list -> 'a list` and
+  `@ : 'a list -> 'a list -> 'a list`. Now `let rec fact …`,
+  `let rec map f xs = match xs with … h :: t -> f h :: map f t`, and
+  `let rec sum …` all infer correctly:
+    `fact : Int -> Int`
+    `len  : 'a list -> Int`
+    `map  : ('a -> 'b) -> 'a list -> 'b list`
+    `sum [1;2;3] : Int`
 - 2026-05-08 Phase 5 — HM constructor inference for option/result (+7
   tests, 351 total). `ocaml-hm-ctor-env` registers None/Some (`'a opt`),
   Ok/Error (`('a, 'b) result`). `:con NAME` instantiates the scheme;