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>

lib/hyperscript/parser.sx

@@ -2506,6 +2506,33 @@
           (let
             ((target (cond ((at-end?) (list (quote beingTold))) ((and (= (tp-type) "keyword") (or (= (tp-val) "then") (= (tp-val) "end"))) (list (quote beingTold))) (true (parse-expr)))))
             (list (quote close-element) target))))
+      (define
+        parse-js-block
+        (fn
+          ()
+          (let
+            ((params (if (= (tp-type) "paren-open") (do (adv!) (define collect-params! (fn (acc) (cond ((or (at-end?) (= (tp-type) "paren-close")) (do (when (= (tp-type) "paren-close") (adv!)) acc)) ((= (tp-type) "comma") (do (adv!) (collect-params! acc))) (true (let ((pname (tp-val))) (do (adv!) (collect-params! (append acc pname)))))))) (collect-params! (list))) (list))))
+            (let
+              ((js-start (cur-start)))
+              (define
+                skip-to-end!
+                (fn
+                  ()
+                  (if
+                    (or
+                      (at-end?)
+                      (and (= (tp-type) "keyword") (= (tp-val) "end")))
+                    nil
+                    (do (adv!) (skip-to-end!)))))
+              (skip-to-end!)
+              (let
+                ((js-end (cur-start)))
+                (let
+                  ((js-src (substring src js-start js-end)))
+                  (when
+                    (and (= (tp-type) "keyword") (= (tp-val) "end"))
+                    (adv!))
+                  (list (quote js-block) params js-src)))))))
       (define
         parse-cmd
         (fn
@@ -2655,6 +2682,8 @@
                 (do (adv!) (list (quote continue))))
               ((and (= typ "keyword") (or (= val "exit") (= val "halt")))
                 (do (adv!) (list (quote exit))))
+              ((and (= typ "keyword") (= val "js"))
+                (do (adv!) (parse-js-block)))
               (true (parse-expr))))))
       (define
         parse-cmd-list
@@ -2710,7 +2739,8 @@
                 (= v "close")
                 (= v "pick")
                 (= v "ask")
-                (= v "answer"))))
+                (= v "answer")
+                (= v "js"))))
           (define
             cl-collect
             (fn