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/boot.sx

@@ -433,7 +433,15 @@
       (run-post-render-hooks)
       (flush-collected-styles)
       (set-timeout (fn () (process-elements nil)) 0)
-      (dom-listen (dom-window) "popstate" (fn (e) (handle-popstate 0)))
+      (dom-listen
+        (dom-window)
+        "popstate"
+        (fn
+          (e)
+          (let
+            ((state (host-get e "state"))
+              (scrollY (if state (or (dict-get state "scrollY") 0) 0)))
+            (handle-popstate scrollY))))
       (dom-set-attr
         (host-get (dom-document) "documentElement")
         "data-sx-ready"