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/asset_loader.rs

@@ -12,10 +12,11 @@
 //! 4. Bevy uploads it to the GPU automatically
 
 use bevy::asset::io::Reader;
-use bevy::asset::{AssetLoader, AsyncReadExt, LoadContext};
+use bevy::asset::{AssetLoader, LoadContext};
+use bevy::image::ImageSampler;
 use bevy::prelude::*;
 use bevy::render::render_asset::RenderAssetUsages;
-use bevy::render::render_resource::{Extent3d, TextureDimension, TextureFormat};
+use bevy::render::render_resource::{Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages};
 use sylpheed_formats::texture::{X360Texture, X360TextureFormat};
 
 // ── Plugin ────────────────────────────────────────────────────────────────────
@@ -86,24 +87,32 @@ impl AssetLoader for Xpr2TextureLoader {
 
 /// Convert a decoded `X360Texture` into a Bevy-compatible `Image`.
 ///
-/// Maps Xbox 360 D3DFORMAT codes to `wgpu::TextureFormat` values.
-/// Bevy uses wgpu internally, so this mapping is direct.
+/// Constructs the `Image` struct directly rather than using `Image::new()`,
+/// because `Image::new()` calls `pixel_size()` which panics for BCn
+/// block-compressed formats.
 pub fn x360_texture_to_bevy_image(tex: X360Texture) -> Result<Image, Xpr2LoadError> {
-    let bevy_format = x360_format_to_wgpu(&tex.format)?;
+    let format = x360_format_to_wgpu(&tex.format)?;
 
-    Ok(Image::new(
-        Extent3d {
-            width: tex.width,
-            height: tex.height,
-            depth_or_array_layers: 1,
+    Ok(Image {
+        data: tex.data,
+        texture_descriptor: TextureDescriptor {
+            label: None,
+            size: Extent3d {
+                width: tex.width,
+                height: tex.height,
+                depth_or_array_layers: 1,
+            },
+            mip_level_count: 1,
+            sample_count: 1,
+            dimension: TextureDimension::D2,
+            format,
+            usage: TextureUsages::TEXTURE_BINDING | TextureUsages::COPY_DST,
+            view_formats: &[],
         },
-        TextureDimension::D2,
-        tex.data,
-        bevy_format,
-        // Make the texture available on both CPU and GPU
-        // (RenderOnly would be more efficient once RE is complete)
-        RenderAssetUsages::MAIN_WORLD | RenderAssetUsages::RENDER_WORLD,
-    ))
+        sampler: ImageSampler::Default,
+        texture_view_descriptor: None,
+        asset_usage: RenderAssetUsages::MAIN_WORLD | RenderAssetUsages::RENDER_WORLD,
+    })
 }
 
 /// Map an `X360TextureFormat` to the corresponding `wgpu::TextureFormat`.
@@ -119,14 +128,12 @@ fn x360_format_to_wgpu(format: &X360TextureFormat) -> Result<TextureFormat, Xpr2
         X360TextureFormat::Dxt3 => TextureFormat::Bc2RgbaUnormSrgb,
         // DXT5 / BC3
         X360TextureFormat::Dxt5 => TextureFormat::Bc3RgbaUnormSrgb,
-        // DXN / BC5 — normal maps (RG, not sRGB)
+        // DXN / BC5 — two-channel normal maps (RG, not sRGB)
         X360TextureFormat::Dxn => TextureFormat::Bc5RgUnorm,
-        // Uncompressed ARGB — note Xbox 360 is BGRA order (big-endian)
-        // We may need to swizzle R and B channels
+        // DXT5A / BC4 — single-channel (gloss, specular, luminance maps)
+        X360TextureFormat::Dxt5A => TextureFormat::Bc4RUnorm,
+        // Uncompressed ARGB — Xbox 360 stores as BGRA in memory
         X360TextureFormat::A8R8G8B8 | X360TextureFormat::X8R8G8B8 => {
-            // Xbox 360 stores as ARGB big-endian (BGRA in memory)
-            // wgpu uses Rgba8UnormSrgb — swizzle may be needed
-            // TODO: verify channel order against actual game textures
             TextureFormat::Bgra8UnormSrgb
         }
     })