Add server architecture essay to sx-docs

Documents the boundary enforcement model: three tiers, boundary types,
runtime validation, the SX-in-Python rule, and the multi-language story.

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

sx/sx/nav-data.sx

@@ -75,6 +75,8 @@
         :summary "Self-reference, and the tangled hierarchy of a language that defines itself.")
   (dict :label "The Reflexive Web" :href "/essays/reflexive-web"
         :summary "A web where pages can inspect, modify, and extend their own rendering pipeline.")
+  (dict :label "Server Architecture" :href "/essays/server-architecture"
+        :summary "How SX enforces the boundary between host and embedded language, and what it looks like across targets.")
   (dict :label "sx sucks" :href "/essays/sx-sucks"
         :summary "An honest accounting of everything wrong with SX and why you probably shouldn't use it.")))