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/page-functions.sx

@@ -674,3 +674,5 @@
 
 (define eval-rules (fn (&key title &rest args) (quasiquote (~geography/eval-rules-content))))
 
+
+(define sxtp (make-page-fn "~applications/sxtp/content" "~applications/sxtp/" nil "-content"))