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/browser/sx-platform.js

@@ -19,9 +19,16 @@
   function boot(K) {
 
   // ================================================================
-  // 8 FFI Host Primitives
+  // FFI Host Primitives
   // ================================================================
 
+  // Lazy module loading — islands/components call this to declare dependencies
+  K.registerNative("load-library!", function(args) {
+    var name = args[0];
+    if (!name) return false;
+    return __sxLoadLibrary(name) || false;
+  });
+
   K.registerNative("host-global", function(args) {
     var name = args[0];
     if (typeof globalThis !== "undefined" && name in globalThis) return globalThis[name];
@@ -463,13 +470,16 @@
       loadLibrary(info.deps[i], loading);
     }
 
+    // Mark as loaded BEFORE executing — self-imports (define-library re-exports)
+    // will see it as already loaded and skip rather than infinite-looping.
+    _loadedLibs[name] = true;
+
     // Load this module
     var ok = loadBytecodeFile("sx/" + info.file);
     if (!ok) {
       var sxFile = info.file.replace(/\.sxbc$/, '.sx');
       ok = loadSxFile("sx/" + sxFile);
     }
-    _loadedLibs[name] = true;
     return !!ok;
   }
 
@@ -614,8 +624,14 @@
     var _doInit = function() {
       loadWebStack();
       Sx.init();
-      // Enable JIT after all boot code has run
-      setTimeout(function() { K.eval('(enable-jit!)'); }, 0);
+      // Enable JIT after all boot code has run.
+      // Lazy-load the compiler first — JIT needs it to compile functions.
+      setTimeout(function() {
+        if (K.beginModuleLoad) K.beginModuleLoad();
+        loadLibrary("sx compiler", {});
+        if (K.endModuleLoad) K.endModuleLoad();
+        K.eval('(enable-jit!)');
+      }, 0);
     };
 
     if (document.readyState === "loading") {

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 -> ());