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/render.sx

@@ -178,7 +178,9 @@
 ;; bindings = ((name1 expr1) (name2 expr2) ...)
 (define process-bindings
   (fn (bindings env)
-    (let ((local (merge env)))
+    ;; env-extend (not merge) — Env is not a dict subclass, so merge()
+    ;; returns an empty dict, losing all parent scope bindings.
+    (let ((local (env-extend env)))
       (for-each
         (fn (pair)
           (when (and (= (type-of pair) "list") (>= (len pair) 2))