HS: fix as Values SELECT + multi-select programmatic changes

- hs-value-of-node: use selectedIndex fallback when SELECT.value is
  empty (mock DOM doesn't auto-compute it from selected options)
- generate-sx-tests: manual body for 'programmatically changed
  selections' test — deselect dog, select cat before reading values

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

lib/hyperscript/runtime.sx

@@ -1180,7 +1180,17 @@
           (if
             (host-get node "multiple")
             (hs-select-multi-values node)
-            (host-get node "value")))
+            (let
+              ((idx (host-get node "selectedIndex"))
+                (opts (host-get node "options"))
+                (raw-val (host-get node "value")))
+              (if
+                (and (not (nil? raw-val)) (not (= raw-val "")))
+                raw-val
+                (if
+                  (and (not (nil? opts)) (>= idx 0))
+                  (host-get (if (list? opts) (nth opts idx) (host-get opts idx)) "value")
+                  "")))))
         ((or (= typ "checkbox") (= typ "radio"))
           (if (host-get node "checked") (host-get node "value") nil))
         (true (host-get node "value"))))))