HS: nil guard in hs-on for missing targets (+1 test)

When `from #doesntExist` resolves to nil, hs-on silently skips
listener registration instead of crashing on dom-listen nil.
Removes "can ignore when target doesn't exist" from skip-list.

Also adds host-make-js-thrower native utility (plain JS throwing
function, no K.callFn re-entry) — investigated for the js-exceptions
catch test but that test stays skipped: native JS throws from host
calls escape OCaml WASM try-with guards.

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

spec/tests/test-hyperscript-behavioral.sx

@@ -9472,7 +9472,14 @@
       (hs-activate! _el-div)
     ))
   (deftest "can ignore when target doesn't exist"
-    (error "SKIP (skip-list): can ignore when target doesn't exist"))
+    (hs-cleanup!)
+    (let ((_el (dom-create-element "div")))
+      (dom-set-attr _el "_" "on click from #doesntExist throw \"bar\" on click put \"clicked\" into me")
+      (dom-append (dom-body) _el)
+      (hs-activate! _el)
+      (dom-dispatch _el "click" nil)
+      (assert= (dom-get-inner-html _el) "clicked"))
+  )
   (deftest "can invoke on multiple events"
     (hs-cleanup!)
     (let ((_el-div (dom-create-element "div")))