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/Cargo.toml

@@ -19,3 +19,6 @@ tracing               = { workspace = true }
 tracing-subscriber    = { workspace = true }
 colored               = "2"
 indicatif             = "0.17"
+# Texture PNG export: BCn software decode + image encode.
+texpresso             = "2"
+image                 = { version = "0.25", default-features = false, features = ["png"] }