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/hs-run-filtered.js

@@ -568,6 +568,41 @@ K.registerNative('host-to-list',([obj])=>{try{return[...obj];}catch(e){return[];
 K.registerNative('host-await',a=>{});
 K.registerNative('load-library!',()=>false);
 
+// ── JS block execution support ─────────────────────────────────
+// Track promise states for synchronous introspection in hs-js-exec
+const _promiseStates = new WeakMap();
+const _origPReject = Promise.reject.bind(Promise);
+const _origPResolve = Promise.resolve.bind(Promise);
+Promise.reject = function(v) {
+  const p = _origPReject(v);
+  _promiseStates.set(p, {ok: false, value: v});
+  p.catch(() => {}); // suppress unhandled rejection warning
+  return p;
+};
+Promise.resolve = function(v) {
+  if (v && typeof v === 'object' && typeof v.then === 'function') return _origPResolve(v);
+  const p = _origPResolve(v);
+  _promiseStates.set(p, {ok: true, value: v});
+  return p;
+};
+
+K.registerNative('host-new-function', a => {
+  const paramList = a[0];
+  const src = a[1];
+  const params = paramList && paramList._type === 'list' && paramList.items
+    ? Array.from(paramList.items)
+    : Array.isArray(paramList) ? paramList : [];
+  try { return new Function(...params, src); } catch(e) { return null; }
+});
+
+K.registerNative('host-promise-state', a => {
+  const p = a[0];
+  if (!p || typeof p.then !== 'function') return null;
+  const s = _promiseStates.get(p);
+  if (!s) return null;
+  return {ok: s.ok, value: s.value};
+});
+
 let _testDeadline = 0;
 // Mock fetch routes
 const _fetchRoutes = {

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")))',
+    ],
 }