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>

tests/playwright/generate-sx-tests.py

@@ -128,6 +128,18 @@ SKIP_TEST_NAMES = {
 # Manually-written SX test bodies for tests whose upstream body cannot be
 # auto-translated. Key = test name; value = SX lines to emit inside deftest.
 MANUAL_TEST_BODIES = {
+    "converts multiple selects with programmatically changed selections": [
+        '    (let ((_node (dom-create-element "form")))',
+        '      (dom-set-inner-html _node "<select name=\"animal\" multiple> <option value=\"dog\" selected>Doggo</option> <option value=\"cat\">Kitteh</option> <option value=\"raccoon\" selected>Trash Panda</option> <option value=\"possum\">Sleepy Boi</option> </select>")',
+        '      (let ((_sel (dom-query _node "select")))',
+        '        (let ((_opts (host-get _sel "options")))',
+        '          (host-set! (nth _opts 0) "selected" false)',
+        '          (host-set! (nth _opts 1) "selected" true)',
+        '          (let ((_result (eval-hs-locals "x as Values" (list (list (quote x) _node)))))',
+        '            (assert= (nth (host-get _result "animal") 0) "cat")',
+        '            (assert= (nth (host-get _result "animal") 1) "raccoon")',
+        '          ))))',
+    ],
     "iterate cookies values work": [
         '    (hs-cleanup!)',
         '    (host-set! (host-global "cookies") "foo" "bar")',