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>

sx/sx/home-stepper.sx

@@ -169,9 +169,7 @@
                        (dom-set-attr sp "class"
                          (str base
                            (cond
-                             (and (= step-num cur) is-spread) " opacity-60"
                              (= step-num cur) " bg-amber-100 rounded px-0.5 font-bold text-sm"
-                             (and (< step-num cur) is-spread) " opacity-60"
                              (< step-num cur) " font-bold text-xs"
                              :else " opacity-40")))))))
                  (range 0 (min (len spans) (len tokens)))))))))
@@ -277,9 +275,7 @@
                     (cls (str (get tok "cls")
                               (cond
                                 (= step -1) ""
-                                (and (= step cur) is-spread) " opacity-60"
                                 (= step cur) " bg-amber-100 rounded px-0.5 font-bold text-sm"
-                                (and (< step cur) is-spread) " opacity-60"
                                 (< step cur) " font-bold text-xs"
                                 :else " opacity-40"))))
                 (span :class cls (get tok "text"))))