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/tests/run.py

@@ -127,7 +127,8 @@ def render_html(sx_source):
     except ImportError:
         raise RuntimeError("render-to-html not available — sx_ref.py not built")
     exprs = parse_all(sx_source)
-    render_env = dict(env)
+    # Use Env (not flat dict) so tests exercise the real scope chain path.
+    render_env = _Env(dict(env))
     result = ""
     for expr in exprs:
         result += _render_to_html(expr, render_env)
@@ -143,7 +144,9 @@ def render_sx(sx_source):
     except ImportError:
         raise RuntimeError("aser not available — sx_ref.py not built")
     exprs = parse_all(sx_source)
-    render_env = dict(env)
+    # Use Env (not flat dict) so tests exercise the real scope chain path.
+    # Using dict(env) hides bugs where merge() drops Env parent scopes.
+    render_env = _Env(dict(env))
     result = ""
     for expr in exprs:
         val = _aser(expr, render_env)