sx-host plan steps 1-2: defhelper + SX config + SXTP spec + nav tools

Step 1 — defhelper: SX-defined page data helpers replace Python helpers.
(defhelper name (params) body) in .sx files, using existing IO primitives
(query, action, service). Loaded into OCaml kernel as pure SX defines.

Step 2 — SX config: app-config.sx replaces app-config.yaml with (defconfig)
form. (env-get "VAR") resolves secrets from environment. Kebab-to-underscore
aliasing ensures backward compatibility with all 174 config consumers.

Also: SXTP protocol spec (applications/sxtp/spec.sx), docs article,
sx_nav move/delete modes, reactive-runtime moved to geography.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/nav-tree.sx

@@ -77,7 +77,11 @@
             :href "/sx/(geography.(cek))"
             :label "CEK Machine"
             :children cek-nav-items)
-          (dict :href "/sx/(geography.(capabilities))" :label "Capabilities")))
+          (dict :href "/sx/(geography.(capabilities))" :label "Capabilities")
+          (dict
+            :href "/sx/(geography.(reactive-runtime))"
+            :label "Reactive Runtime"
+            :children reactive-runtime-nav-items)))
       (dict
         :href "/sx/(language)"
         :label "Language"
@@ -115,13 +119,10 @@
             :label "Protocols"
             :children protocols-nav-items)
           (dict :href "/sx/(applications.(sx-pub))" :label "sx-pub")
-          (dict
-            :href "/sx/(applications.(reactive-runtime))"
-            :label "Reactive Runtime"
-            :children reactive-runtime-nav-items)
           (dict
             :href "/sx/(applications.(native-browser))"
-            :label "Native Browser")))
+            :label "Native Browser")
+          (dict :href "/sx/(applications.(sxtp))" :label "SXTP Protocol")))
       (dict :href "/sx/(tools)" :label "Tools" :children tools-nav-items)
       (dict
         :href "/sx/(etc)"