HS: E39 WebWorker plugin stub (+1 test)

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

lib/hyperscript/parser.sx

@@ -2749,6 +2749,7 @@
               ((= val "behavior") (do (adv!) (parse-behavior-feat)))
               ((= val "live") (do (adv!) (parse-live-feat)))
               ((= val "when") (do (adv!) (parse-when-feat)))
+              ((= val "worker") (error "worker plugin is not installed — see https://hyperscript.org/features/worker"))
               (true (parse-cmd-list))))))
       (define
         coll-feats

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

@@ -2749,6 +2749,7 @@
               ((= val "behavior") (do (adv!) (parse-behavior-feat)))
               ((= val "live") (do (adv!) (parse-live-feat)))
               ((= val "when") (do (adv!) (parse-when-feat)))
+              ((= val "worker") (error "worker plugin is not installed — see https://hyperscript.org/features/worker"))
               (true (parse-cmd-list))))))
       (define
         coll-feats

spec/tests/test-hyperscript-behavioral.sx

@@ -13595,5 +13595,9 @@ end")
 ;; ── worker (1 tests) ──
 (defsuite "hs-upstream-worker"
   (deftest "raises a helpful error when the worker plugin is not installed"
-    (error "SKIP (untranslated): raises a helpful error when the worker plugin is not installed"))
+    (let ((result (guard (e (true (if (string? e) e (str e))))
+                    (hs-compile "worker MyWorker def noop() end end")
+                    "")))
+      (assert (contains? result "worker plugin"))
+      (assert (contains? result "hyperscript.org/features/worker"))))
 )