Add production build pipeline (#46)

## Summary
- split development dynamic linking from production builds
- add headless processed-asset generation
- add a separate production CI cache and static Bevy/Rust linkage verification
- keep custom debug features enabled for production builds behind a feature gate

Reviewed-on: #46
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/bin/process-assets.rs

@@ -0,0 +1,31 @@
+use bevy::{
+    app::ScheduleRunnerPlugin,
+    asset::{AssetMode, AssetPlugin, processor::AssetProcessor, processor::ProcessorState},
+    prelude::*,
+    tasks::block_on,
+};
+use shinespark::AssetProcessingPlugin;
+use std::{process::ExitCode, time::Duration};
+
+fn main() -> ExitCode {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins.set(ScheduleRunnerPlugin::run_loop(Duration::from_millis(50))))
+        .add_plugins(AssetPlugin {
+            mode: AssetMode::Processed,
+            use_asset_processor_override: Some(true),
+            ..default()
+        })
+        .add_plugins(AssetProcessingPlugin)
+        .add_systems(Update, finish_when_processed);
+
+    app.run();
+    ExitCode::SUCCESS
+}
+
+fn finish_when_processed(processor: Option<Res<AssetProcessor>>, mut exit: MessageWriter<AppExit>) {
+    if processor
+        .is_some_and(|processor| block_on(processor.get_state()) == ProcessorState::Finished)
+    {
+        exit.write(AppExit::Success);
+    }
+}