make it clear how to use module system without mach.Core (remove mach.App)

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

examples/play-opus/App.zig

@@ -19,8 +19,8 @@ pub const name = .app;
 pub const Mod = mach.Mod(@This());
 
 pub const systems = .{
+    .start = .{ .handler = start },
     .init = .{ .handler = init },
-    .after_init = .{ .handler = afterInit },
     .deinit = .{ .handler = deinit },
     .tick = .{ .handler = tick },
     .audio_state_change = .{ .handler = audioStateChange },
@@ -32,15 +32,28 @@ pub const components = .{
 
 sfx: mach.Audio.Opus,
 
-fn init(
-    entities: *mach.Entities.Mod,
+fn start(
     core: *mach.Core.Mod,
     audio: *mach.Audio.Mod,
     app: *Mod,
 ) !void {
     core.schedule(.init);
     audio.schedule(.init);
-    app.schedule(.after_init);
+    app.schedule(.init);
+}
+
+fn init(
+    entities: *mach.Entities.Mod,
+    core: *mach.Core.Mod,
+    audio: *mach.Audio.Mod,
+    app: *Mod,
+) !void {
+    core.state().on_tick = app.system(.tick);
+    core.state().on_exit = app.system(.deinit);
+
+    // Configure the audio module to send our app's .audio_state_change event when an entity's sound
+    // finishes playing.
+    audio.state().on_state_change = app.system(.audio_state_change);
 
     const bgm_fbs = std.io.fixedBufferStream(assets.bgm.bit_bit_loop);
     const bgm_sound_stream = std.io.StreamSource{ .const_buffer = bgm_fbs };
@@ -68,12 +81,6 @@ fn init(
     core.schedule(.start);
 }
 
-fn afterInit(audio: *mach.Audio.Mod, app: *Mod) void {
-    // Configure the audio module to send our app's .audio_state_change event when an entity's sound
-    // finishes playing.
-    audio.state().on_state_change = app.system(.audio_state_change);
-}
-
 fn deinit(core: *mach.Core.Mod, audio: *mach.Audio.Mod) void {
     audio.schedule(.deinit);
     core.schedule(.deinit);

examples/play-opus/main.zig

@@ -1,17 +1,25 @@
+const std = @import("std");
 const mach = @import("mach");
 
-// The global list of Mach modules registered for use in our application.
+// The global list of Mach modules our application may use.
 pub const modules = .{
     mach.Core,
     mach.Audio,
     @import("App.zig"),
 };
 
-// TODO(important): use standard entrypoint instead
+// TODO: move this to a mach "entrypoint" zig module which handles nuances like WASM requires.
 pub fn main() !void {
-    // Initialize mach.Core
-    try mach.core.initModule();
+    const allocator = std.heap.c_allocator;
 
-    // Main loop
-    while (try mach.core.tick()) {}
+    // Initialize module system
+    try mach.mods.init(allocator);
+
+    // Schedule .app.start to run.
+    mach.mods.schedule(.app, .start);
+
+    // Dispatch systems forever or until there are none left to dispatch. If your app uses mach.Core
+    // then this will block forever and never return.
+    const stack_space = try allocator.alloc(u8, 8 * 1024 * 1024);
+    try mach.mods.dispatch(stack_space, .{});
 }