fix(texture): correct Xenos de-tile + endian + cubemaps + PNG export

Reworks the XPR2 texture path after RE against the retail disc (viewer
previews were scrambled/failed).

- de-tile: replace the naive Morton-over-32×32 approximation with the
  faithful Xenos Tiled2D bank/pipe/macro-tile address formula (ported
  from xenia texture_address.h). Verified correct: the Acheron backdrop
  decodes to a sharp planet with its spiral storm.
- endian: undo the X360 word byte-swap per the fetch-constant endianness
  field (k8in16/k8in32/k16in32) — without it BCn/ARGB data is noise.
- cubemaps: parse TXCM resources (skybox/backdrops), decoding face 0,
  instead of erroring "No TX2D found".
- A8R8G8B8: correct channel order after the k8in32 swap ([A,R,G,B]→RGBA)
  — the Acheron backdrop is now correctly green, not red.
- XPR files are texture PACKS; add XPR_RES_INDEX to select a resource.
- CLI `texture export` now writes real PNGs (texpresso BCn decode +
  image), replacing the stub. Adds texpresso + image deps.
- tests/texture_disc.rs: integration test running the pipeline over real
  disc .xpr files (28/28 parse). RE debug knobs: XPR_NO_DETILE /
  XPR_NO_ENDIAN / XPR_FORCE_ENDIAN.

KNOWN ISSUE: mipmapped DXT1 textures still decode to noise — mip 0 is
not at base_address in the multi-resource packs (localized to a mip
storage-offset quirk; uncompressed + de-tile + endian are all verified).

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

crates/sylpheed-cli/src/main.rs

@@ -330,44 +330,68 @@ fn cmd_texture_export(file: &Path, output: &Path) -> Result<()> {
     let bytes = std::fs::read(file)
         .with_context(|| format!("Cannot read {}", file.display()))?;
 
-    use sylpheed_formats::texture::{X360Texture, X360TextureFormat};
+    use sylpheed_formats::texture::X360Texture;
     let tex = X360Texture::from_xpr2(&bytes)?;
+    let rgba = decode_to_rgba8(&tex)
+        .with_context(|| format!("decoding {:?} texture", tex.format))?;
 
-    // For BCn compressed textures we need to software-decompress to RGBA8
-    // before saving to PNG. This uses the `texpresso` crate.
-    //
-    // TODO: add `texpresso = "2"` to Cargo.toml for BCn software decode.
-    //       For now, print a helpful message.
-    match tex.format {
-        X360TextureFormat::A8R8G8B8 | X360TextureFormat::X8R8G8B8 => {
-            // Raw BGRA8 — can save directly (with channel swizzle)
-            println!(
-                "{} TODO: save raw BGRA8 to PNG (add `image` crate)",
-                "Note:".yellow()
-            );
-        }
-        compressed_format => {
-            println!(
-                "{} Format {:?} needs BCn decompression before PNG export.",
-                "Note:".yellow(), compressed_format
-            );
-            println!(
-                "  Add `texpresso` crate and implement decompression in texture.rs"
-            );
-            println!(
-                "  Alternatively, use the Bevy viewer to inspect textures visually."
-            );
-        }
-    }
+    image::save_buffer(
+        output,
+        &rgba,
+        tex.width,
+        tex.height,
+        image::ExtendedColorType::Rgba8,
+    )
+    .with_context(|| format!("writing PNG {}", output.display()))?;
 
     println!(
-        "  Texture parsed OK: {}×{} {:?}",
-        tex.width, tex.height, tex.format
+        "{} {}×{} {:?}{} → {}",
+        "Exported".green().bold(),
+        tex.width,
+        tex.height,
+        tex.format,
+        if tex.is_cubemap { " (cubemap face 0)" } else { "" },
+        output.display().to_string().cyan(),
     );
-
     Ok(())
 }
 
+/// Software-decode a de-tiled `X360Texture` (mip 0) to tightly-packed RGBA8.
+///
+/// BCn blocks are decompressed with `texpresso`; uncompressed A8R8G8B8/X8R8G8B8
+/// is byte-swizzled from the Xenos in-memory BGRA order.
+fn decode_to_rgba8(tex: &sylpheed_formats::texture::X360Texture) -> Result<Vec<u8>> {
+    use sylpheed_formats::texture::X360TextureFormat as F;
+    let (w, h) = (tex.width as usize, tex.height as usize);
+    let mut rgba = vec![0u8; w * h * 4];
+
+    let bc = |fmt: texpresso::Format, rgba: &mut [u8]| {
+        fmt.decompress(&tex.data, w, h, rgba);
+    };
+
+    match tex.format {
+        F::Dxt1 => bc(texpresso::Format::Bc1, &mut rgba),
+        F::Dxt3 => bc(texpresso::Format::Bc2, &mut rgba),
+        F::Dxt5 => bc(texpresso::Format::Bc3, &mut rgba),
+        F::A8R8G8B8 | F::X8R8G8B8 => {
+            // After the k8in32 endian swap in from_xpr2, k_8_8_8_8 pixels are in
+            // [A,R,G,B] byte order (verified against the retail Acheron backdrop).
+            // Emit RGBA. X8 has no meaningful alpha.
+            let opaque = matches!(tex.format, F::X8R8G8B8);
+            for (px, out) in tex.data.chunks_exact(4).zip(rgba.chunks_exact_mut(4)) {
+                out[0] = px[1]; // R
+                out[1] = px[2]; // G
+                out[2] = px[3]; // B
+                out[3] = if opaque { 0xFF } else { px[0] };
+            }
+        }
+        other => {
+            anyhow::bail!("PNG export for {other:?} (BC4/BC5) not implemented yet");
+        }
+    }
+    Ok(rgba)
+}
+
 // ── pak list ────────────────────────────────────────────────────────────────
 
 /// Best-effort human label for a decompressed entry's inner format.