Invert event loop

lib/rt/jsrt.h

@@ -10,7 +10,15 @@
 // str.h      strings and the string natives
 // number.h   the number natives
 // any.h      any_t and JS's operators on it
-// io.h       IO natives, async on io_uring (uring.h)
+// io.h       IO natives, async on io_uring (uring.h), and the host's loop
+//
+// The host owns the event loop. The program exports two fns, which the host
+// calls, and a flavor of the runtime is the host. This one, for Linux, is the
+// `main` in io.h: define JSRT_NO_HOST to bring another
+//
+// jsrt_start()      runs the program's top level, then its microtasks
+// jsrt_complete(h)  op `h` is done: runs its callback, then the microtasks.
+//                   The op's result can be read only until this returns
 #pragma once
 
 // io.h needs Linux's syscall() and mmap(), which -std=c11 hides. This must
@@ -20,6 +28,12 @@
 #endif
 
 #include "core.h"
+
+#include <stdint.h>
+
+void jsrt_start(void);
+void jsrt_complete(int32_t h);
+
 #include "str.h"
 #include "number.h"
 #include "any.h"