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/eval.sx

@@ -162,6 +162,7 @@
               (= name "reset")       (sf-reset args env)
               (= name "shift")       (sf-shift args env)
               (= name "dynamic-wind") (sf-dynamic-wind args env)
+              (= name "scope")      (sf-scope args env)
               (= name "provide")    (sf-provide args env)
 
               ;; Higher-order forms
@@ -951,11 +952,35 @@
 
 
 ;; --------------------------------------------------------------------------
-;; 6a2. provide — render-time dynamic scope
+;; 6a2. scope — unified render-time dynamic scope primitive
 ;; --------------------------------------------------------------------------
 ;;
-;; (provide name value body...) — push a named scope with value and empty
-;; accumulator, evaluate body, pop scope. Returns last body result.
+;; (scope name body...) or (scope name :value v body...)
+;; Push a named scope with optional value and empty accumulator,
+;; evaluate body, pop scope. Returns last body result.
+;;
+;; `provide` is sugar: (provide name value body...) = (scope name :value value body...)
+
+(define sf-scope
+  (fn ((args :as list) (env :as dict))
+    (let ((name (trampoline (eval-expr (first args) env)))
+          (rest (slice args 1))
+          (val nil)
+          (body-exprs nil))
+      ;; Check for :value keyword
+      (if (and (>= (len rest) 2) (= (type-of (first rest)) "keyword") (= (keyword-name (first rest)) "value"))
+        (do (set! val (trampoline (eval-expr (nth rest 1) env)))
+            (set! body-exprs (slice rest 2)))
+        (set! body-exprs rest))
+      (scope-push! name val)
+      (let ((result nil))
+        (for-each (fn (e) (set! result (trampoline (eval-expr e env)))) body-exprs)
+        (scope-pop! name)
+        result))))
+
+
+;; provide — sugar for scope with a value
+;; (provide name value body...) → (scope name :value value body...)
 
 (define sf-provide
   (fn ((args :as list) (env :as dict))
@@ -963,9 +988,9 @@
           (val (trampoline (eval-expr (nth args 1) env)))
           (body-exprs (slice args 2))
           (result nil))
-      (provide-push! name val)
+      (scope-push! name val)
       (for-each (fn (e) (set! result (trampoline (eval-expr e env)))) body-exprs)
-      (provide-pop! name)
+      (scope-pop! name)
       result)))