HS: js-block return values + worker stub test

Parser: parse-js-block extracts raw JS source by character positions.
Compiler: js-block AST → hs-js-exec call, stores result in it.
Runtime: hs-js-exec creates JS Function, handles promise rejection.
Test runner: host-new-function/host-promise-state natives + promise monkey-patch.

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

spec/tests/test-hyperscript-behavioral.sx

@@ -13636,5 +13636,12 @@ 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"))
+    (hs-cleanup!)
+    (let ((caught nil))
+      (guard (_e (true (set! caught (str _e))))
+        (hs-compile "worker MyWorker def noop() end end"))
+      (assert (not (nil? caught)))
+      (assert (string-contains? caught "worker plugin"))
+      (assert (string-contains? caught "hyperscript.org/features/worker")))
+  )
 )