SX request handler + AJAX nav + shared JIT globals + shell cleanup

- Remove prism.js, sweetalert2, body.js, sx-browser.js from shell —
  only WASM kernel (sx_browser.bc.wasm.js + sx-platform.js) loads
- Restore request-handler.sx integration: SX handles routing + AJAX
  detection, OCaml does aser → SSR → shell render pipeline
- AJAX fragment support: SX-Request header returns content fragment
  (~14KB) instead of full page (~858KB), cached with "ajax:" prefix
- Fix language/applications/etc page functions to return empty fragment
  instead of nil (was causing 404s)
- Shared JIT VM globals: env_bind hook mirrors ALL bindings to a single
  shared globals table — eliminates stale-snapshot class of JIT bugs
- Add native `parse` function for components that need SX parsing
- Clean up unused shell params (sx-js-hash, body-js-hash, head-scripts,
  body-scripts, use-wasm) from shell.sx, helpers.py, and server.ml

14/32 Playwright tests pass (navigation, SSR, isomorphic, geography).
Remaining failures are client-side (WASM bytecode 404s block hydration).

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

sx/sx/page-functions.sx

@@ -24,7 +24,7 @@
     (content)
     (if (nil? content) (quote (~docs-content/home-content)) content)))
 
-(define language (fn (content) (if (nil? content) nil content)))
+(define language (fn (content) (if (nil? content) (quote (<>)) content)))
 
 (define
   geography
@@ -32,9 +32,11 @@
     (content)
     (if (nil? content) (quote (~geography/index-content)) content)))
 
-(define applications (fn (content) (if (nil? content) nil content)))
+(define
+  applications
+  (fn (content) (if (nil? content) (quote (<>)) content)))
 
-(define etc (fn (content) (if (nil? content) nil content)))
+(define etc (fn (content) (if (nil? content) (quote (<>)) content)))
 
 (define hypermedia (fn (content) (if (nil? content) nil content)))
 
@@ -668,11 +670,24 @@
     "/plan-"
     "-content"))
 
-(define capabilities (fn (&key title &rest args) (quasiquote (~geography/capabilities-content))))
+(define
+  capabilities
+  (fn
+    (&key title &rest args)
+    (quasiquote (~geography/capabilities-content))))
 
-(define modules (fn (&key title &rest args) (quasiquote (~geography/modules-content))))
+(define
+  modules
+  (fn (&key title &rest args) (quasiquote (~geography/modules-content))))
 
-(define eval-rules (fn (&key title &rest args) (quasiquote (~geography/eval-rules-content))))
+(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"))
+(define
+  sxtp
+  (make-page-fn
+    "~applications/sxtp/content"
+    "~applications/sxtp/"
+    nil
+    "-content"))