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/boundary.sx

@@ -374,30 +374,44 @@
 
 
 ;; --------------------------------------------------------------------------
-;; Tier 5: Dynamic scope — render-time provide/context/emit!
+;; Tier 5: Scoped effects — unified render-time dynamic scope
 ;;
-;; `provide` is a special form (not a primitive) that creates a named scope
-;; with a value and an empty accumulator. `context` reads the value from the
-;; nearest enclosing provider. `emit!` appends to the accumulator, `emitted`
-;; reads the accumulated values.
+;; `scope` is the general primitive. `provide` is sugar for scope-with-value.
+;; Both `provide` and `scope` are special forms in the evaluator.
 ;;
-;; The platform must implement per-name stacks. Each entry has a value and
-;; an emitted list. `provide-push!`/`provide-pop!` manage the stack.
+;; The platform must implement per-name stacks. Each entry has a value,
+;; an emitted list, and a dedup flag. `scope-push!`/`scope-pop!` manage
+;; the stack. `provide-push!`/`provide-pop!` are aliases.
+;;
+;; `collect!`/`collected`/`clear-collected!` (Tier 4) are backed by scopes:
+;; collect! lazily creates a root scope with dedup=true, then emits into it.
 ;; --------------------------------------------------------------------------
 
-(declare-tier :dynamic-scope :source "eval.sx")
+(declare-tier :scoped-effects :source "eval.sx")
+
+(declare-spread-primitive "scope-push!"
+  :params (name value)
+  :returns "nil"
+  :effects [mutation]
+  :doc "Push a scope with name and value. General form — provide-push! is an alias.")
+
+(declare-spread-primitive "scope-pop!"
+  :params (name)
+  :returns "nil"
+  :effects [mutation]
+  :doc "Pop the most recent scope for name. General form — provide-pop! is an alias.")
 
 (declare-spread-primitive "provide-push!"
   :params (name value)
   :returns "nil"
   :effects [mutation]
-  :doc "Push a provider scope with name and value (platform internal).")
+  :doc "Alias for scope-push!. Push a scope with name and value.")
 
 (declare-spread-primitive "provide-pop!"
   :params (name)
   :returns "nil"
   :effects [mutation]
-  :doc "Pop the most recent provider scope for name (platform internal).")
+  :doc "Alias for scope-pop!. Pop the most recent scope for name.")
 
 (declare-spread-primitive "context"
   :params (name &rest default)