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/Cargo.toml

@@ -31,11 +31,14 @@ bevy = { workspace = true, features = [
     "bevy_winit",
     "multi_threaded",
     "png",
+    # Required for TonyMcMapFace tonemapping (default Camera3d tonemapper)
+    "tonemapping_luts",
     # Linux display backends
     "x11",
     "wayland",
 ] }
 bevy_egui  = { workspace = true }
+futures    = { workspace = true }
 
 thiserror  = { workspace = true }
 tracing    = { workspace = true }
@@ -46,3 +49,4 @@ dev = ["bevy/dynamic_linking"]
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 tracing-subscriber = { workspace = true }
+rfd                = { workspace = true }