Unify scoped effects: scope as general primitive, provide as sugar

- Add `scope` special form to eval.sx: (scope name body...) or
  (scope name :value v body...) — general dynamic scope primitive
- `provide` becomes sugar: (provide name value body...) calls scope
- Rename provide-push!/provide-pop! to scope-push!/scope-pop! throughout
  all adapters (async, dom, html, sx) and platform implementations
- Update boundary.sx: Tier 5 now "Scoped effects" with scope-push!/
  scope-pop! as primary, provide-push!/provide-pop! as aliases
- Add scope form handling to async adapter and aser wire format
- Update sx-browser.js, sx_ref.py (bootstrapped output)
- Add scopes.sx docs page, update provide/spreads/demo docs
- Update nav-data, page-functions, docs page definitions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/js.sx

@@ -527,6 +527,8 @@
   "collect!" "sxCollect"
   "collected" "sxCollected"
   "clear-collected!" "sxClearCollected"
+  "scope-push!" "scopePush"
+  "scope-pop!" "scopePop"
   "provide-push!" "providePush"
   "provide-pop!" "providePop"
   "context" "sxContext"