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/platform_js.py

@@ -1194,7 +1194,7 @@ PLATFORM_JS_PRE = '''
 
   // JSON / dict helpers for island state serialization
   function jsonSerialize(obj) {
-    try { return JSON.stringify(obj); } catch(e) { return "{}"; }
+    return JSON.stringify(obj);
   }
   function isEmptyDict(d) {
     if (!d || typeof d !== "object") return true;
@@ -1204,11 +1204,34 @@ PLATFORM_JS_PRE = '''
 
   function envHas(env, name) { return name in env; }
   function envGet(env, name) { return env[name]; }
-  function envSet(env, name, val) { env[name] = val; }
+  function envSet(env, name, val) {
+    // Walk prototype chain to find where the variable is defined (for set!)
+    var obj = env;
+    while (obj !== null && obj !== Object.prototype) {
+      if (obj.hasOwnProperty(name)) { obj[name] = val; return; }
+      obj = Object.getPrototypeOf(obj);
+    }
+    // Not found in any parent scope — set on the immediate env
+    env[name] = val;
+  }
   function envExtend(env) { return Object.create(env); }
   function envMerge(base, overlay) {
+    // Same env or overlay is descendant of base — just extend, no copy.
+    // This prevents set! inside lambdas from modifying shadow copies.
+    if (base === overlay) return Object.create(base);
+    var p = overlay;
+    for (var d = 0; p && p !== Object.prototype && d < 100; d++) {
+      if (p === base) return Object.create(base);
+      p = Object.getPrototypeOf(p);
+    }
+    // General case: extend base, copy ONLY overlay properties that don't
+    // exist in the base chain (avoids shadowing closure bindings).
     var child = Object.create(base);
-    if (overlay) for (var k in overlay) if (overlay.hasOwnProperty(k)) child[k] = overlay[k];
+    if (overlay) {
+      for (var k in overlay) {
+        if (overlay.hasOwnProperty(k) && !(k in base)) child[k] = overlay[k];
+      }
+    }
     return child;
   }
 
@@ -1649,8 +1672,11 @@ PLATFORM_DOM_JS = """
   function domListen(el, name, handler) {
     if (!_hasDom || !el) return function() {};
     // Wrap SX lambdas from runtime-evaluated island code into native fns
+    // If lambda takes 0 params, call without event arg (convenience for on-click handlers)
     var wrapped = isLambda(handler)
-      ? function(e) { try { invoke(handler, e); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } }
+      ? (lambdaParams(handler).length === 0
+        ? function(e) { try { invoke(handler); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } }
+        : function(e) { try { invoke(handler, e); } catch(err) { console.error("[sx-ref] domListen handler error:", name, err); } })
       : handler;
     if (name === "click") logInfo("domListen: click on <" + (el.tagName||"?").toLowerCase() + "> text=" + (el.textContent||"").substring(0,20) + " isLambda=" + isLambda(handler));
     el.addEventListener(name, wrapped);