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/render.sx

@@ -269,11 +269,13 @@
 ;;   (collected bucket)        → list
 ;;   (clear-collected! bucket) → void
 ;;
-;; Dynamic scope (provide/context/emit!):
-;;   (provide-push! name val)  → void
-;;   (provide-pop! name)       → void
-;;   (context name &rest def)  → value from nearest provider
-;;   (emit! name value)        → void (append to provider accumulator)
+;; Scoped effects (scope/provide/context/emit!):
+;;   (scope-push! name val)    → void (general form)
+;;   (scope-pop! name)         → void (general form)
+;;   (provide-push! name val)  → alias for scope-push!
+;;   (provide-pop! name)       → alias for scope-pop!
+;;   (context name &rest def)  → value from nearest scope
+;;   (emit! name value)        → void (append to scope accumulator)
 ;;   (emitted name)            → list of emitted values
 ;;
 ;; From parser.sx: