render-dom-lake: mark reused lakes to prevent SPA nav conflicts

After reusing an SSR lake element, set data-sx-lake-claimed attribute.
Subsequent dom-query uses :not([data-sx-lake-claimed]) to skip already-
reused elements, preventing SPA navigation from picking up stale lakes
from previous pages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/wasm/sx/boot-helpers.sx

@@ -501,7 +501,10 @@
                   "#sx-content")))
             (if
               (try-client-route (url-pathname href) target-sel)
-              (do (browser-push-state nil "" href) (browser-scroll-to 0 0))
+              (do
+                (save-scroll-position)
+                (browser-push-state nil "" href)
+                (browser-scroll-to 0 0))
               (do
                 (log-info (str "sx:route server fetch " href))
                 (dom-set-attr link "sx-get" href)