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/orchestration.sx

@@ -527,6 +527,7 @@
                   (if
                     client-routed
                     (do
+                      (save-scroll-position)
                       (browser-push-state (get live-info "url"))
                       (browser-scroll-to 0 0))
                     (do
@@ -1553,11 +1554,23 @@
                       (dom-dispatch el event-name detail))))))))
         els))))
 
+(define
+  save-scroll-position
+  :effects (io)
+  (fn
+    ()
+    (let
+      ((scrollY (host-get (dom-window) "scrollY")))
+      (browser-replace-state
+        (dict "scrollY" scrollY)
+        ""
+        (browser-location-href)))))
+
 (define
   handle-popstate
   :effects (mutation io)
   (fn
-    ((scrollY :as number))
+    (scrollY)
     (let
       ((url (browser-location-href))
         (boost-el (dom-query "[sx-boost]"))
@@ -1566,9 +1579,8 @@
             boost-el
             (let
               ((attr (dom-get-attr boost-el "sx-boost")))
-              (if (and attr (not (= attr "true"))) attr nil))
-            nil))
-        (target-sel (or target-sel "#main-panel"))
+              (if (and attr (not (= attr "true"))) attr "#sx-content"))
+            "#sx-content"))
         (target (dom-query target-sel))
         (pathname (url-pathname url)))
       (when
@@ -1577,7 +1589,7 @@
           (try-client-route pathname target-sel)
           (browser-scroll-to 0 scrollY)
           (let
-            ((headers (build-request-headers target (loaded-component-names) _css-hash)))
+            ((headers (build-request-headers target "GET" url)))
             (fetch-and-restore target url headers scrollY)))))))
 
 (define