Fix process-bindings scope loss and async-invoke arity, bootstrap async adapter

Two bugs fixed:
1. process-bindings used merge(env) which returns {} for Env objects
   (Env is not a dict subclass). Changed to env-extend in render.sx
   and adapter-async.sx. This caused "Undefined symbol: theme" etc.
2. async-aser-eval-call passed evaled-args list to async-invoke(&rest),
   double-wrapping it. Changed to inline apply + coroutine check.

Also: bootstrap define-async into sx_ref.py (Phase 6), replace ~1000 LOC
hand-written async_eval_ref.py with 24-line thin re-export shim.

Test runner now uses Env (not flat dict) for render envs to catch scope bugs.
8 new regression tests (4 scope chain, 2 native callable arity, 2 render).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/js.sx

@@ -1290,8 +1290,9 @@
           (= name "append!")
             (str (js-expr (nth expr 1)) ".push(" (js-expr (nth expr 2)) ");")
           (= name "env-set!")
-            (str (js-expr (nth expr 1)) "[" (js-expr (nth expr 2))
-                 "] = " (js-expr (nth expr 3)) ";")
+            (str "envSet(" (js-expr (nth expr 1))
+                 ", " (js-expr (nth expr 2))
+                 ", " (js-expr (nth expr 3)) ");")
           (= name "set-lambda-name!")
             (str (js-expr (nth expr 1)) ".name = " (js-expr (nth expr 2)) ";")
           :else