Add provide/context/emit!/emitted — render-time dynamic scope

Four new primitives for scoped downward value passing and upward
accumulation through the render tree. Specced in .sx, bootstrapped
to Python and JS across all adapters (eval, html, sx, dom, async).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/adapter-dom.sx

@@ -359,7 +359,7 @@
   (list "if" "when" "cond" "case" "let" "let*" "begin" "do"
         "define" "defcomp" "defisland" "defmacro" "defstyle" "defhandler"
         "map" "map-indexed" "filter" "for-each" "portal"
-        "error-boundary"))
+        "error-boundary" "provide"))
 
 (define render-dom-form? :effects []
   (fn ((name :as string))
@@ -598,6 +598,19 @@
             coll)
           frag)
 
+      ;; provide — render-time dynamic scope
+      (= 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)
+          (for-each
+            (fn (i)
+              (dom-append frag (render-to-dom (nth expr i) env ns)))
+            (range 3 (len expr)))
+          (provide-pop! prov-name)
+          frag)
+
       ;; Fallback
       :else
         (render-to-dom (trampoline (eval-expr expr env)) env ns))))