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>

Cargo.toml

@@ -39,6 +39,9 @@ anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
+# ── Native file dialog ────────────────────────────────────────────────────────
+rfd = "0.14"
+
 # ── CLI ───────────────────────────────────────────────────────────────────────
 clap = { version = "4", features = ["derive"] }