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>

sx/sxc/pages/docs.sx

@@ -617,6 +617,12 @@
 ;; Provide / Emit! section (under Geography)
 ;; ---------------------------------------------------------------------------
 
+(defpage scopes-index
+  :path "/geography/scopes/"
+  :auth :public
+  :layout :sx-docs
+  :content (~layouts/doc :path "/sx/(geography.(scopes))" (~geography/scopes-content)))
+
 (defpage provide-index
   :path "/geography/provide/"
   :auth :public