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/spreads.sx

@@ -89,8 +89,9 @@
     (p :class "text-stone-500 text-sm italic mb-8"
       "A spread is a value that, when returned as a child of an element, "
       "injects attributes onto its parent instead of rendering as content. "
-      "Internally, spreads work through " (code "provide") "/" (code "emit!") " — "
-      "every element creates a provider scope, and spread children emit into it.")
+      "Internally, spreads work through "
+      (a :href "/sx/(geography.(scopes))" :class "text-violet-600 hover:underline" "scopes")
+      " — every element creates a scope, and spread children emit into it.")
 
     ;; =====================================================================
     ;; I. How it works
@@ -125,9 +126,9 @@
     ;; =====================================================================
 
     (~docs/section :title "collect! — the other upward channel" :id "collect"
-      (p "Spreads use " (code "provide") "/" (code "emit!") " (scoped, no dedup). "
-         (code "collect!") "/" (code "collected") " is a separate upward channel — "
-         "global, with automatic deduplication. Used for CSS rule accumulation.")
+      (p "Spreads use " (code "scope") "/" (code "emit!") " (scoped, no dedup). "
+         (code "collect!") "/" (code "collected") " is also backed by scopes — "
+         "a lazy root scope with automatic deduplication. Used for CSS rule accumulation.")
       (~docs/code :code (highlight ";; Deep inside a component tree:\n(collect! \"cssx\" \".sx-bg-red-500{background:red}\")\n\n;; At the flush point (once, in the layout):\n(let ((rules (collected \"cssx\")))\n  (clear-collected! \"cssx\")\n  (raw! (str \"<style>\" (join \"\" rules) \"</style>\")))" "lisp"))
       (p "Both are upward communication through the render tree, but with different "
          "semantics — " (code "emit!") " is scoped to the nearest provider, "