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/compiler.sx

@@ -2268,6 +2268,25 @@
                   (list (quote hs-halt!) (quote event) (nth ast 1)))
                 ((= head (quote focus!))
                   (list (quote dom-focus) (hs-to-sx (nth ast 1))))
+                ((= head (quote js-block))
+                  (let
+                    ((params (nth ast 1)) (js-src (nth ast 2)))
+                    (let
+                      ((bound-syms (map (fn (p) (make-symbol p)) params)))
+                      (list
+                        (quote let)
+                        (list
+                          (list
+                            (quote __hs-js)
+                            (list
+                              (quote hs-js-exec)
+                              (cons (quote list) params)
+                              js-src
+                              (cons (quote list) bound-syms))))
+                        (list
+                          (quote begin)
+                          (list (quote set!) (quote it) (quote __hs-js))
+                          (quote __hs-js))))))
                 (true ast)))))))))
 
 ;; ── Convenience: source → SX ─────────────────────────────────