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-html.sx

@@ -433,11 +433,11 @@
 
         ;; Render the island body as HTML
         (let ((body-html (render-to-html (component-body island) local))
-              (state-json (serialize-island-state kwargs)))
+              (state-sx (serialize-island-state kwargs)))
           ;; Wrap in container with hydration attributes
           (str "<span data-sx-island=\"" (escape-attr island-name) "\""
-               (if state-json
-                 (str " data-sx-state=\"" (escape-attr state-json) "\"")
+               (if state-sx
+                 (str " data-sx-state=\"" (escape-attr state-sx) "\"")
                  "")
                ">"
                body-html
@@ -445,17 +445,17 @@
 
 
 ;; --------------------------------------------------------------------------
-;; serialize-island-state — serialize kwargs to JSON for hydration
+;; serialize-island-state — serialize kwargs to SX for hydration
 ;; --------------------------------------------------------------------------
 ;;
-;; Only serializes simple values (numbers, strings, booleans, nil, lists, dicts).
-;; Functions, components, and other non-serializable values are skipped.
+;; Uses the SX serializer (not JSON) so the client can parse with sx-parse.
+;; Handles all SX types natively: numbers, strings, booleans, nil, lists, dicts.
 
 (define serialize-island-state
   (fn (kwargs)
     (if (empty-dict? kwargs)
       nil
-      (json-serialize kwargs))))
+      (sx-serialize kwargs))))
 
 
 ;; --------------------------------------------------------------------------
@@ -476,8 +476,8 @@
 ;; Raw HTML construction:
 ;;   (make-raw-html s)          → wrap string as raw HTML (not double-escaped)
 ;;
-;; JSON serialization (for island state):
-;;   (json-serialize dict)      → JSON string
+;; Island state serialization:
+;;   (sx-serialize val)         → SX source string (from parser.sx)
 ;;   (empty-dict? d)            → boolean
 ;;   (escape-attr s)            → HTML attribute escape
 ;;