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/el-compiler/src/codegen.el

@@ -2502,6 +2502,7 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "http_post_with_headers") { return 3 }
     if str_eq(name, "http_post_form_auth") { return 3 }
     if str_eq(name, "http_serve") { return 2 }
+    if str_eq(name, "http_serve_async") { return 2 }
     if str_eq(name, "http_set_handler") { return 1 }
     // Seed primitives (__-prefix) — runtime/el_seed.c
     if str_eq(name, "__str_len") { return 1 }