Step 7a complete: ->> |> as-> pipe operators + transpiler fixes

Three new threading operators in evaluator.sx:
- ->> (thread-last): inserts value as last arg
- |> (pipe): alias for ->> (F#/OCaml convention)
- as-> (thread-anywhere): binds value to named variable

  (->> 10 (- 3))           ;; => -7  (thread-last: (- 3 10))
  (-> 10 (- 3))            ;; => 7   (thread-first: (- 10 3))
  (->> 1 (list 2 3))       ;; => (2 3 1)
  (as-> 5 x (+ x 1) (* x 2)) ;; => 12

Two transpiler bugs fixed:
1. Non-recursive functions (let without rec) weren't chained as `and`
   in the let rec block — became local bindings inside previous function
2. CekFrame "extra" field wasn't in the cf_extra key mapping — mode
   was always Nil, making thread-last fall through to thread-first

Also: added missing step-sf-case definition to evaluator.

2644 tests pass, zero regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_ref.ml

@@ -111,7 +111,7 @@ and make_case_frame match_val remaining env =
 
 (* make-thread-frame *)
 and make_thread_frame remaining env mode name =
-  (CekFrame { cf_type = "thread"; cf_env = env; cf_name = name; cf_body = Nil; cf_remaining = remaining; cf_f = Nil; cf_args = Nil; cf_results = Nil; cf_extra = Nil; cf_extra2 = Nil })
+  (CekFrame { cf_type = "thread"; cf_env = env; cf_name = name; cf_body = Nil; cf_remaining = remaining; cf_f = Nil; cf_args = Nil; cf_results = Nil; cf_extra = mode; cf_extra2 = Nil })
 
 (* thread-insert-arg *)
 and thread_insert_arg form value fenv =