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/sx/nav-data.sx

@@ -374,10 +374,12 @@
           :children (list
             {:label "Reference" :href "/sx/(geography.(hypermedia.(reference)))" :children reference-nav-items}
             {:label "Examples" :href "/sx/(geography.(hypermedia.(example)))" :children examples-nav-items})}
+        {:label "Scopes" :href "/sx/(geography.(scopes))"
+          :summary "The unified primitive beneath provide, collect!, spreads, and islands. Named scope with downward value, upward accumulation, and a dedup flag."}
         {:label "Provide / Emit!" :href "/sx/(geography.(provide))"
-          :summary "Render-time dynamic scope — the substrate beneath spreads, CSSX, and script collection. Downward context, upward accumulation, one mechanism."}
+          :summary "Sugar for scope-with-value. Render-time dynamic scope — the substrate beneath spreads, CSSX, and script collection."}
         {:label "Spreads" :href "/sx/(geography.(spreads))"
-          :summary "Child-to-parent communication across render boundaries — spread, collect!, reactive-spread, built on provide/emit!."}
+          :summary "Child-to-parent communication across render boundaries — spread, collect!, reactive-spread, built on scopes."}
         {:label "Marshes" :href "/sx/(geography.(marshes))"
           :summary "Where reactivity and hypermedia interpenetrate — server writes to signals, reactive transforms reshape server content, client state modifies how hypermedia is interpreted."}
         {:label "Isomorphism" :href "/sx/(geography.(isomorphism))" :children isomorphism-nav-items})}