HS: socket on-message + as JSON (+3)

Steps 4-5 complete: hs-try-json-parse, ws.onmessage wiring (text/JSON
dispatch), onmessage test cases. 8/16 socket tests passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/static/wasm/sx/hs-runtime.sx

@@ -2528,6 +2528,10 @@
 
 ;; ── WebSocket / socket feature ───────────────────────────────────
 
+(define
+  hs-try-json-parse
+  (fn (s) (host-call (host-global "JSON") "parse" s)))
+
 (define
   hs-socket-register!
   (fn
@@ -2553,6 +2557,27 @@
                 wrapper
                 "rpc"
                 (host-call proxy-factory "call" nil wrapper))))
+          (host-set!
+            ws
+            "onmessage"
+            (host-callback
+              (fn
+                (event)
+                (let
+                  ((data (host-get event "data")))
+                  (let
+                    ((parsed (hs-try-json-parse data)))
+                    (cond
+                      ((and (not (nil? parsed)) (not (nil? (host-get parsed "iid"))))
+                        nil)
+                      ((not (nil? handler))
+                        (if
+                          json?
+                          (if
+                            (not (nil? parsed))
+                            (handler parsed)
+                            (error "Received non-JSON message"))
+                          (handler event)))))))))
           (define
             bind-path!
             (fn