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-html.sx

@@ -56,7 +56,7 @@
   (list "if" "when" "cond" "case" "let" "let*" "begin" "do"
         "define" "defcomp" "defisland" "defmacro" "defstyle" "defhandler"
         "deftype" "defeffect"
-        "map" "map-indexed" "filter" "for-each"))
+        "map" "map-indexed" "filter" "for-each" "provide"))
 
 (define render-html-form? :effects []
   (fn ((name :as string))
@@ -237,6 +237,21 @@
                     (render-to-html (apply f (list item)) env)))
                 coll))))
 
+      ;; 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)))
+              (body-start 3)
+              (body-count (- (len expr) 3)))
+          (provide-push! prov-name prov-val)
+          (let ((result (if (= body-count 1)
+                          (render-to-html (nth expr body-start) env)
+                          (join "" (filter (fn (r) (not (spread? r)))
+                                    (map (fn (i) (render-to-html (nth expr i) env))
+                                         (range body-start (+ body-start body-count))))))))
+            (provide-pop! prov-name)
+            result))
+
       ;; Fallback
       :else
         (render-value-to-html (trampoline (eval-expr expr env)) env))))