feat(viewer): world cubemap (skybox) viewer — 6-face grid

TXCM XPR2 resources are the game's world skyboxes (BG_Acheron, BG_Hargenteen,
…). The viewer showed only face 0 as a flat 2D image; now it decodes all 6 faces
and shows a labelled grid.

Formats (Bevy-free): X360Texture::cube_faces_from_xpr2() returns a Cubemap with 6
faces in D3D9 order (+X −X +Y −Y +Z −Z), or None for ordinary 2D textures. Face
layout derived from xenia's GetGuestTextureLayout — 6 back-to-back independently-
tiled surfaces, per-face stride = tiled-surface-size aligned to the 4 KiB
subresource boundary (kTextureSubresourceAlignmentBytes). Verified against real
BG_Acheron: data_size == 6 × 0x400000, and all 6 faces decode cleanly (own
Python decode + a disc test asserting 6×4 MiB faces and face 0 == the validated
from_xpr2 green-planet decode). Extracted a shared decode_surface() helper so the
2D and cube paths are byte-identical; from_xpr2 output unchanged (re-verified).

Viewer: new SkyboxPreview resource (6 egui face textures, reusing the existing
per-format x360_texture_to_bevy_image); populated in apply_loaded_texture's TXCM
branch; freed/reset alongside the other previews (factored free_texture/
free_skybox helpers). Central panel gains a skybox branch rendering a 3-column
labelled face grid.

DEFERRED (per "do not guess, else defer"): the interactive 3D skybox. Face data +
D3D9 order are validated, but wgpu cube-sampling handedness can't be confirmed
without eyeballing the GUI — a wrong-oriented skybox is worse than the correct
labelled grid. The grid is the reliable deliverable; the 3D look-around is a
follow-up once orientation is visually confirmed.

(Background agent did the investigation/validation but was blocked from writing
files; implemented here in the main tree from its findings, independently
re-verified.)

23 formats tests + disc cubemap test pass; viewer/CLI build; face-0 export
re-verified as the green planet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/sylpheed-formats/tests/pak_idxd_disc.rs

@@ -6,6 +6,7 @@
 
 use std::path::{Path, PathBuf};
 
+use sylpheed_formats::texture::X360Texture;
 use sylpheed_formats::{IdxdObject, PakArchive};
 
 /// Locate the extracted disc root, or `None` to skip.
@@ -122,3 +123,25 @@ fn name_lookup_resolves_weapon_tbl() {
     let obj = IdxdObject::parse(&bytes).expect("weapon.tbl parses as IDXD");
     assert!(obj.count > 0);
 }
+
+/// The Acheron backdrop is a TXCM world cubemap: 6 faces, and the cube path's
+/// face 0 matches the (validated) 2D `from_xpr2` face-0 decode.
+#[test]
+fn acheron_world_cubemap_six_faces() {
+    skip_without_disc!(root);
+    let bytes = std::fs::read(root.join("hidden/resource3d/BG_Acheron.xpr")).unwrap();
+
+    let cube = X360Texture::cube_faces_from_xpr2(&bytes)
+        .expect("cube decode ok")
+        .expect("BG_Acheron is a TXCM cubemap");
+    assert_eq!((cube.width, cube.height), (1024, 1024));
+    assert_eq!(cube.faces.len(), 6);
+    // A8R8G8B8 → 4 bytes/texel, de-tiled to width×height.
+    for face in &cube.faces {
+        assert_eq!(face.len(), 1024 * 1024 * 4);
+    }
+    // Face 0 must equal what the existing 2D path decodes (the green planet).
+    let face0_2d = X360Texture::from_xpr2(&bytes).unwrap();
+    assert!(face0_2d.is_cubemap);
+    assert_eq!(cube.faces[0], face0_2d.data);
+}