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/adapter-dom.sx

@@ -181,7 +181,7 @@
           (el (dom-create-element tag new-ns)))
 
       ;; Provide scope for spread emit! — deeply nested spreads emit here
-      (provide-push! "element-attrs" nil)
+      (scope-push! "element-attrs" nil)
 
       ;; Process args: keywords → attrs, others → children
       (reduce
@@ -269,7 +269,7 @@
                     (dom-set-attr el key (str val))))))
             (keys spread-dict)))
         (emitted "element-attrs"))
-      (provide-pop! "element-attrs")
+      (scope-pop! "element-attrs")
 
       el)))
 
@@ -381,7 +381,7 @@
   (list "if" "when" "cond" "case" "let" "let*" "begin" "do"
         "define" "defcomp" "defisland" "defmacro" "defstyle" "defhandler"
         "map" "map-indexed" "filter" "for-each" "portal"
-        "error-boundary" "provide"))
+        "error-boundary" "scope" "provide"))
 
 (define render-dom-form? :effects []
   (fn ((name :as string))
@@ -637,17 +637,39 @@
             coll)
           frag)
 
-      ;; provide — render-time dynamic scope
+      ;; scope — unified render-time dynamic scope
+      (= name "scope")
+        (let ((scope-name (trampoline (eval-expr (nth expr 1) env)))
+              (rest-args (slice expr 2))
+              (scope-val nil)
+              (body-exprs nil)
+              (frag (create-fragment)))
+          ;; Check for :value keyword
+          (if (and (>= (len rest-args) 2)
+                   (= (type-of (first rest-args)) "keyword")
+                   (= (keyword-name (first rest-args)) "value"))
+            (do (set! scope-val (trampoline (eval-expr (nth rest-args 1) env)))
+                (set! body-exprs (slice rest-args 2)))
+            (set! body-exprs rest-args))
+          (scope-push! scope-name scope-val)
+          (for-each
+            (fn (e)
+              (dom-append frag (render-to-dom e env ns)))
+            body-exprs)
+          (scope-pop! scope-name)
+          frag)
+
+      ;; provide — sugar for scope with value
       (= name "provide")
         (let ((prov-name (trampoline (eval-expr (nth expr 1) env)))
               (prov-val (trampoline (eval-expr (nth expr 2) env)))
               (frag (create-fragment)))
-          (provide-push! prov-name prov-val)
+          (scope-push! prov-name prov-val)
           (for-each
             (fn (i)
               (dom-append frag (render-to-dom (nth expr i) env ns)))
             (range 3 (len expr)))
-          (provide-pop! prov-name)
+          (scope-pop! prov-name)
           frag)
 
       ;; Fallback