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/adapter-dom.sx

@@ -186,15 +186,10 @@
                       (attr-expr (nth args (inc (get state "i")))))
                   (cond
                     ;; Event handler: evaluate eagerly, bind listener
-                    ;; If handler is a 0-arity lambda, wrap to ignore the event arg
                     (starts-with? attr-name "on-")
                       (let ((attr-val (trampoline (eval-expr attr-expr env))))
                         (when (callable? attr-val)
-                          (dom-listen el (slice attr-name 3)
-                            (if (and (lambda? attr-val)
-                                     (= (len (lambda-params attr-val)) 0))
-                              (fn (e) (call-lambda attr-val (list) (lambda-closure attr-val)))
-                              attr-val))))
+                          (dom-listen el (slice attr-name 3) attr-val)))
                     ;; Two-way input binding: :bind signal
                     (= attr-name "bind")
                       (let ((attr-val (trampoline (eval-expr attr-expr env))))