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/test-render.sx

@@ -149,7 +149,20 @@
 
   (deftest "let in render context"
     (assert-equal "<p>hello</p>"
-      (render-html "(let ((x \"hello\")) (p x))"))))
+      (render-html "(let ((x \"hello\")) (p x))")))
+
+  (deftest "let preserves outer scope bindings"
+    ;; Regression: process-bindings must preserve parent env scope chain.
+    ;; Using merge() on Env objects returns empty dict (Env is not dict subclass).
+    (assert-equal "<p>outer</p>"
+      (render-html "(do (define theme \"outer\") (let ((x 1)) (p theme)))")))
+
+  (deftest "nested let preserves outer scope"
+    (assert-equal "<div><span>hello</span><span>world</span></div>"
+      (render-html "(do (define a \"hello\")
+                        (define b \"world\")
+                        (div (let ((x 1)) (span a))
+                             (let ((y 2)) (span b))))"))))
 
 
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