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>

tests/playwright/generate-sx-tests.py

@@ -155,6 +155,15 @@ MANUAL_TEST_BODIES = {
         '      (assert-contains "foo" _names)',
         '      (assert-contains "bar" _values))',
     ],
+    "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")))',
+    ],
 }