OCaml VM browser: safe equality, thunk trampolining, platform functions, nav pipeline

Core runtime fixes:
- Safe equality (=, !=): physical equality for dicts/lambdas/signals,
  structural only for acyclic types. Prevents infinite loops on circular
  signal subscriber chains.
- contains?: same safe comparison (physical first, structural for simple types)
- Thunk trampolining in as_number and to_string: leaked thunks auto-resolve
  instead of showing <thunk> or erroring "Expected number, got thunk"
- Diagnostic first error: shows actual type received

Island hydration fixes:
- adapter-dom.sx: skip scope-emit for spreads inside islands (was tripling classes)
- schedule-idle: wrap callback to absorb requestIdleCallback deadline arg
- home-stepper: remove spread-specific highlighting (all tokens same style per step)

Platform functions (boot-helpers.sx):
- fetch-request: 3-arg interface (config, success-fn, error-fn) with promise chain
- build-request-body: form serialization for GET/POST
- strip-component-scripts / extract-response-css: SX text processing
- Navigation: bind-boost-link, bind-client-route-click via execute-request
- Loading state: show-indicator, disable-elements, clear-loading-state
- DOM extras: dom-remove, dom-attr-list (name/value pairs), dom-child-list (SX list),
  dom-is-active-element?, dom-is-input-element?, dom-is-child-of?, dom-on,
  dom-parse-html-document, dom-body-inner-html, create-script-clone
- All remaining stubs: csrf-token, loaded-component-names, observe-intersection,
  event-source-connect/listen, with-transition, cross-origin?, etc.

Navigation pipeline:
- browser-push-state/replace-state: accept 1-arg (URL only) or 3-arg
- boot.sx: wire popstate listener to handle-popstate
- URL updates working via handle-history + pushState fix

Morph debugging (WIP):
- dom-child-list returns proper SX list (was JS Array)
- dom-query accepts optional root element for scoped queries
- Navigation fetches and renders SX responses, URL updates, but morph
  doesn't replace content div (investigating dom-child-list on new elements)

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

web/lib/browser.sx

@@ -26,13 +26,24 @@
   (fn (url)
     (starts-with? url (browser-location-origin))))
 
+;; Extract pathname from a URL string using the URL API
+(define url-pathname
+  (fn (url)
+    (host-get (host-new "URL" url (browser-location-origin)) "pathname")))
+
 (define browser-push-state
-  (fn (state title url)
-    (host-call (host-get (dom-window) "history") "pushState" state title url)))
+  (fn (url-or-state &rest rest)
+    (if (empty? rest)
+      ;; Single arg: just URL
+      (host-call (host-get (dom-window) "history") "pushState" nil "" url-or-state)
+      ;; Three args: state, title, url
+      (host-call (host-get (dom-window) "history") "pushState" url-or-state (first rest) (nth rest 1)))))
 
 (define browser-replace-state
-  (fn (state title url)
-    (host-call (host-get (dom-window) "history") "replaceState" state title url)))
+  (fn (url-or-state &rest rest)
+    (if (empty? rest)
+      (host-call (host-get (dom-window) "history") "replaceState" nil "" url-or-state)
+      (host-call (host-get (dom-window) "history") "replaceState" url-or-state (first rest) (nth rest 1)))))
 
 (define browser-reload
   (fn ()
@@ -182,7 +193,7 @@
 
 (define schedule-idle
   (fn (f)
-    (let ((cb (host-callback f)))
+    (let ((cb (host-callback (fn (_deadline) (f)))))
       (if (host-get (dom-window) "requestIdleCallback")
         (host-call (dom-window) "requestIdleCallback" cb)
         (set-timeout cb 0)))))