feat(viewer): ISO / extracted-dir loading pipeline + native file dialog

WIP: add iso_loader.rs — bridges the async XisoReader / synchronous
GameAssets to Bevy's ECS via background threads + mpsc channels polled
per frame (no main-thread blocking); UI open-iso/open-dir/file-select
events → texture preview. Register IsoLoaderPlugin; wire asset_loader
and the egui UI. Deps: rfd (native file dialog, workspace + viewer),
futures, and bevy tonemapping_luts feature.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/sylpheed-viewer/src/lib.rs

@@ -20,6 +20,7 @@ use bevy_egui::EguiPlugin;
 
 pub mod asset_loader;
 pub mod camera;
+pub mod iso_loader;
 pub mod ui;
 
 // ── Application state ────────────────────────────────────────────────────────
@@ -75,6 +76,7 @@ pub fn run() {
 
     app.add_plugins(EguiPlugin);
     app.add_plugins(asset_loader::SylpheedAssetPlugin);
+    app.add_plugins(iso_loader::IsoLoaderPlugin);
     app.add_plugins(camera::OrbitCameraPlugin);
     app.add_plugins(ui::ViewerUiPlugin);