self-review 2026-05-23: ISE recent-first ordering + http_serve_async builtin

Two improvements:

1. ISE scan ordering — engram_scan_nodes_by_type_json now sorts InternalStateEvent
   nodes by created_at DESC (most-recent-first) instead of salience DESC. Old
   high-salience ISEs (session-start, wm-promotion) no longer dominate offset 0,
   burying recent heartbeat and curiosity_scan events at offsets 20000+. New
   behavior: ?limit=10 returns the 10 most recent ISEs regardless of salience.
   All other node types retain existing salience-sorted behavior.

2. http_serve_async registered as elc builtin — added to builtin_arity() in both
   codegen.el (EL compiler source) and dist/platform/elc.c (compiled C). Also
   rebuilt elc binary from updated elc.c. This closes the fragile-patch gap from
   2026-05-21: elc previously treated http_serve_async as an unknown identifier,
   and the gap description noted elc would 'silently revert to blocking http_serve'
   on next soul rebuild. Now http_serve_async has a proper 2-arg arity entry and
   will survive all future soul recompiles without a manual neuron.c patch.

lang/dist/platform/elc.c

@@ -4342,6 +4342,9 @@ el_val_t builtin_arity(el_val_t name) {
   if (str_eq(name, EL_STR("http_serve"))) {
     return 2;
   }
+  if (str_eq(name, EL_STR("http_serve_async"))) {
+    return 2;
+  }
   if (str_eq(name, EL_STR("http_set_handler"))) {
     return 1;
   }