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/render.sx

@@ -269,6 +269,13 @@
 ;;   (collected bucket)        → list
 ;;   (clear-collected! bucket) → void
 ;;
+;; Dynamic scope (provide/context/emit!):
+;;   (provide-push! name val)  → void
+;;   (provide-pop! name)       → void
+;;   (context name &rest def)  → value from nearest provider
+;;   (emit! name value)        → void (append to provider accumulator)
+;;   (emitted name)            → list of emitted values
+;;
 ;; From parser.sx:
 ;;   (sx-serialize val)        → SX source string (aliased as serialize above)
 ;; --------------------------------------------------------------------------