SPA nav improvements: scroll restoration, popstate, history spec

- boot.sx: popstate handler extracts scrollY from history state
- engine.sx: pass scroll position to handle-popstate
- boot-helpers.sx: scroll position tracking in navigation
- orchestration.sx: scroll state management for back/forward nav
- history.spec.js: new Playwright spec for history navigation

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

web/lib/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)