refactor(formats): rework X360 texture descriptor + xiso reader

WIP: restructure texture.rs (X360TextureDesc accessors / decode path) and
adjust the xiso reader. Builds clean (sylpheed-formats).

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

crates/sylpheed-formats/src/xiso.rs

@@ -14,25 +14,48 @@ use std::path::Path;
 
 use anyhow::{Context, Result};
 use tracing::{debug, info};
+use xdvdfs::blockdev::OffsetWrapper;
 use xdvdfs::layout::VolumeDescriptor;
 
 /// A handle to an open XISO image.
-pub struct XisoReader<F: Read + Seek> {
+///
+/// The inner file is wrapped in an [`OffsetWrapper`] which probes the four
+/// known XGD partition offsets (raw XISO, XGD1, XGD2, XGD3) at open time,
+/// so both single-layer raw dumps **and** full dual-layer disc images are
+/// supported transparently.
+pub struct XisoReader<F: Read + Seek + Send + Sync> {
     volume: VolumeDescriptor,
-    file: F,
+    /// Offset-aware block device — all sector reads are shifted by the
+    /// detected partition start offset automatically.
+    file: OffsetWrapper<F, std::io::Error>,
 }
 
 impl<F: Read + Seek + Send + Sync + 'static> XisoReader<F> {
-    pub async fn open(mut file: F) -> Result<Self> {
-        let volume = xdvdfs::read::read_volume(&mut file)
+    pub async fn open(file: F) -> Result<Self> {
+        // OffsetWrapper::new probes four known partition offsets:
+        //   0x00000000  — raw XISO (trimmed, sector 0 = XDVDFS start)
+        //   0x183E0000  — XGD1 (original Xbox)
+        //   0x0FD90000  — XGD2 (Xbox 360, most retail titles)
+        //   0x02080000  — XGD3 (Xbox 360, later dual-layer titles)
+        let mut wrapper = OffsetWrapper::new(file).await.map_err(|e| {
+            anyhow::anyhow!(
+                "No valid XDVDFS partition found in this disc image. \
+                 Tried raw XISO, XGD1, XGD2, and XGD3 offsets. \
+                 Is this a valid Xbox 360 (or original Xbox) disc image? \
+                 (internal error: {e:?})"
+            )
+        })?;
+
+        // Re-read the volume descriptor via the wrapper (now at the correct offset).
+        let volume = xdvdfs::read::read_volume(&mut wrapper)
             .await
-            .context("Failed to read XDVDFS volume descriptor. Is this a valid Xbox 360 ISO?")?;
+            .context("Found XDVDFS partition but failed to parse volume descriptor")?;
 
         info!(
             "Opened XISO: root directory table at sector {}",
             { let s = volume.root_table.region.sector; s }
         );
-        Ok(Self { volume, file })
+        Ok(Self { volume, file: wrapper })
     }
 
     /// List all files in the disc image, recursively (directories excluded).
@@ -135,10 +158,13 @@ impl<F: Read + Seek + Send + Sync + 'static> XisoReader<F> {
     }
 }
 
-/// Open an XISO from a file path (the common case).
+/// Open a disc image from a file path.
+///
+/// Accepts raw XISO dumps and full XGD1/XGD2/XGD3 disc images — the correct
+/// partition offset is detected automatically.
 pub async fn open_iso(path: &Path) -> Result<XisoReader<std::fs::File>> {
     let file = std::fs::File::open(path)
-        .with_context(|| format!("Cannot open ISO: {}", path.display()))?;
+        .with_context(|| format!("Cannot open disc image: {}", path.display()))?;
     XisoReader::open(file).await
 }