Add provide/emit! geography article, update spreads article, fix foundations rendering

- New geography article (provide.sx): four primitives, demos, nested scoping,
  adapter comparison, spec explorer links
- Updated spreads article section VI: provide/emit! is now implemented, not planned
- Fixed foundations.sx: ~docs/code-block → ~docs/code (undefined component
  was causing the page to silently fail to render)
- Added nav entry and defpage route for provide/emit! article

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

sx/sx/nav-data.sx

@@ -374,8 +374,10 @@
           :children (list
             {:label "Reference" :href "/sx/(geography.(hypermedia.(reference)))" :children reference-nav-items}
             {:label "Examples" :href "/sx/(geography.(hypermedia.(example)))" :children examples-nav-items})}
+        {:label "Provide / Emit!" :href "/sx/(geography.(provide))"
+          :summary "Render-time dynamic scope — the substrate beneath spreads, CSSX, and script collection. Downward context, upward accumulation, one mechanism."}
         {:label "Spreads" :href "/sx/(geography.(spreads))"
-          :summary "Child-to-parent communication across render boundaries — spread, collect!, reactive-spread, and the path to provide/context/emit!."}
+          :summary "Child-to-parent communication across render boundaries — spread, collect!, reactive-spread, built on provide/emit!."}
         {:label "Marshes" :href "/sx/(geography.(marshes))"
           :summary "Where reactivity and hypermedia interpenetrate — server writes to signals, reactive transforms reshape server content, client state modifies how hypermedia is interpreted."}
         {:label "Isomorphism" :href "/sx/(geography.(isomorphism))" :children isomorphism-nav-items})}