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-viewer/src/ui.rs

@@ -10,8 +10,8 @@ use bevy::prelude::*;
 use bevy_egui::{egui, EguiContexts};
 
 use crate::iso_loader::{
-    FileInfo, FileSelected, IsoState, PakView, RequestOpenDir, RequestOpenIso, TextPreview,
-    TexturePreview, IsoLoaderSystemSet,
+    FileInfo, FileSelected, IsoState, PakView, RequestOpenDir, RequestOpenIso, SkyboxPreview,
+    TextPreview, TexturePreview, IsoLoaderSystemSet,
 };
 use crate::ViewerState;
 
@@ -131,6 +131,7 @@ fn draw_viewer_ui(
     preview: Res<TexturePreview>,
     text_preview: Res<TextPreview>,
     mut pak_view: ResMut<PakView>,
+    skybox: Res<SkyboxPreview>,
     file_info: Res<FileInfo>,
     mut open_iso_events: EventWriter<RequestOpenIso>,
     mut open_dir_events: EventWriter<RequestOpenDir>,
@@ -265,6 +266,8 @@ fn draw_viewer_ui(
                         ui.label(format!("Loading {name}…"));
                     });
                 });
+            } else if !skybox.faces.is_empty() {
+                draw_skybox_grid(ui, &skybox);
             } else if pak_view.loaded {
                 draw_pak_browser(ui, &mut pak_view);
             } else if let Some(text) = &text_preview.content {
@@ -485,3 +488,40 @@ fn draw_pak_browser(ui: &mut egui::Ui, pak: &mut PakView) {
         pak.selected = new_selection;
     }
 }
+
+// ── World cubemap (skybox) viewer ─────────────────────────────────────────────
+
+/// Show a world cubemap's 6 faces in a labelled grid (D3D9 order). A true
+/// interactive skybox is a follow-up — the face data + order are validated, but
+/// the wgpu cube-sampling handedness needs visual confirmation first.
+fn draw_skybox_grid(ui: &mut egui::Ui, skybox: &SkyboxPreview) {
+    ui.horizontal(|ui| {
+        ui.heading("World cubemap");
+        ui.separator();
+        ui.label(&skybox.info);
+    });
+    ui.label("6 cube faces, D3D9 order (+X −X +Y −Y +Z −Z).");
+    ui.separator();
+
+    egui::ScrollArea::both().show(ui, |ui| {
+        const CELL: f32 = 240.0;
+        egui::Grid::new("cube_faces")
+            .num_columns(3)
+            .spacing([10.0, 10.0])
+            .show(ui, |ui| {
+                for (i, face) in skybox.faces.iter().enumerate() {
+                    ui.vertical(|ui| {
+                        ui.strong(face.label);
+                        let aspect = face.height as f32 / face.width.max(1) as f32;
+                        ui.add(egui::Image::new(egui::load::SizedTexture::new(
+                            face.egui_id,
+                            [CELL, CELL * aspect],
+                        )));
+                    });
+                    if (i + 1) % 3 == 0 {
+                        ui.end_row();
+                    }
+                }
+            });
+    });
+}