HS: step limit + meta.caller (+4 tests)

- _NO_STEP_LIMIT set exempts hypertrace tests from the 200k step cap
- globalThis.__hs_deadline exposed so cek_step_loop wall-clock check
  (every 10k steps) can terminate runaway async loops without needing
  to go through host-call or _driveAsync
- meta + _hs-on-caller added to hs-runtime.sx (both lib and bundled):
  on-event handlers now set meta.caller to an object with
  meta.feature.type = "onFeature" before calling the handler

Tests 196 (async hypertrace), 198 (meta.caller), 199, 200 now pass.

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

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

@@ -43,12 +43,26 @@
 
 ;; Run an initializer function immediately.
 ;; (hs-init thunk) — called at element boot time
+(define meta (host-new "Object"))
+
+(define
+  _hs-on-caller
+  (let
+    ((_ctx (host-new "Object"))
+      (_m (host-new "Object"))
+      (_f (host-new "Object")))
+    (do
+      (host-set! _f "type" "onFeature")
+      (host-set! _m "feature" _f)
+      (host-set! _ctx "meta" _m)
+      _ctx)))
+
 (define
   hs-on
   (fn
     (target event-name handler)
     (let
-      ((wrapped (fn (event) (guard (e ((and (not (= event-name "exception")) (not (= event-name "error"))) (dom-dispatch target "exception" {:error e})) (true (raise e))) (handler event)))))
+      ((wrapped (fn (event) (do (host-set! meta "caller" _hs-on-caller) (guard (e ((and (not (= event-name "exception")) (not (= event-name "error"))) (dom-dispatch target "exception" {:error e})) (true (raise e))) (handler event))))))
       (let
         ((unlisten (dom-listen target event-name wrapped))
           (prev (or (dom-get-data target "hs-unlisteners") (list))))