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>

tests/playwright/generate-sx-tests.py

@@ -109,10 +109,8 @@ SKIP_TEST_NAMES = {
     "can be in a top level script tag",
     "multiple event handlers at a time are allowed to execute with the every keyword",
     "each behavior installation has its own event queue",
-    "can catch exceptions thrown in js functions",
     "can catch exceptions thrown in hyperscript functions",
-    "can ignore when target doesn't exist",
-    "can ignore when target doesn\\'t exist",
+    "can catch exceptions thrown in js functions",
     "can handle an or after a from clause",
     # upstream 'fetch' category — real DocumentFragment semantics (`its childElementCount`
     # after `as html`) not exercisable with our DOM mock.
@@ -202,6 +200,16 @@ MANUAL_TEST_BODIES = {
     "basic classRef works w no match": [
         '    (assert= (len (eval-hs ".badClassThatDoesNotHaveAnyElements")) 0)',
     ],
+    # on from: if target resolves to nil, hs-on silently skips registration
+    "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"))',
+    ],
     # bootstrap: restore correct bodies that auto-regen gets wrong
     "can call functions": [
         '    (hs-cleanup!)',