Lazy module loading: compiler loads on demand, playground page

- load-library! native: islands can declare module dependencies at
  hydration time, triggering on-demand .sxbc loading
- JIT compiler lazy-load: compiler.sxbc loads via setTimeout after boot,
  eliminating "JIT: compiler not loaded" errors
- _import_hook on sx_types: infrastructure for hosts to resolve import
  suspensions inside eval_expr (server wiring deferred to Step 8)
- Playground page (/sx/(tools.(playground))): REPL island that lazy-loads
  the compiler module when navigated to — demonstrates the full
  lazy loading pipeline

Known remaining issues:
- SPA navigation broken for pages using let-match (orchestration.sx,
  router.sx) — bytecode compiler doesn't handle let-match special form
- Server-side "IO suspension in non-IO context" during http_load_files —
  needs cek_run import handling (deferred to Step 8)

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

hosts/ocaml/lib/sx_types.ml

@@ -255,6 +255,10 @@ let _env_bind_hook : (env -> string -> value -> unit) option ref = ref None
    Used by browser kernel to sync mutations back to global_env. *)
 let _vm_global_set_hook : (string -> value -> unit) option ref = ref None
 
+(* Optional hook: called by cek_run on import suspension.
+   If set, the hook loads the library and returns true; cek_run then resumes. *)
+let _import_hook : (value -> bool) option ref = ref None
+
 let env_bind env name v =
   Hashtbl.replace env.bindings (intern name) v;
   (match !_env_bind_hook with Some f -> f env name v | None -> ());