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/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 "provide")    (sf-provide args env)
 
               ;; Higher-order forms
               (= name "map")          (ho-map args env)
@@ -949,6 +950,25 @@
         result))))
 
 
+;; --------------------------------------------------------------------------
+;; 6a2. provide — render-time dynamic scope
+;; --------------------------------------------------------------------------
+;;
+;; (provide name value body...) — push a named scope with value and empty
+;; accumulator, evaluate body, pop scope. Returns last body result.
+
+(define sf-provide
+  (fn ((args :as list) (env :as dict))
+    (let ((name (trampoline (eval-expr (first args) env)))
+          (val (trampoline (eval-expr (nth args 1) env)))
+          (body-exprs (slice args 2))
+          (result nil))
+      (provide-push! name val)
+      (for-each (fn (e) (set! result (trampoline (eval-expr e env)))) body-exprs)
+      (provide-pop! name)
+      result)))
+
+
 ;; --------------------------------------------------------------------------
 ;; 6b. Macro expansion
 ;; --------------------------------------------------------------------------