feat(formats,viewer): decode XBG7 meshes + 3D model preview

Reverse-engineer the single-stream XBG7 geometry layout (clean-room: hex
inspection + geometric validation of the retail disc's
hidden/resource3d/*.xpr, no game code copied) and present models as
textured 3D meshes in the explorer.

Format (docs/re/structures/xbg7-mesh.md): XBG7 geometry resources sit
inside XPR2 containers alongside TX2D textures. For ~25 single-stream
models (weapons, simple props) the data section is a sequence of
sub-meshes, each an u16-BE triangle-list index buffer followed (after a
fixed 12-byte header) by a stride-24 vertex buffer whose declaration is
in the descriptor: POSITION f32x3 @0x00, NORMAL f16x4 @0x0C, TEXCOORD
f16x2 @0x14. Sub-mesh (vtx,idx) counts come from descriptor tuples.

The +12 vertex offset is pinned by the recovered normals being exactly
unit-length (align16 lands 4 bytes early and silently corrupts every
field). A safety gate rejects any model whose indices are out of range
or whose mean |normal| is not ~1, declining garbage (Stage_S*
placeholders, the complex multi-stream hero-ship body) rather than
mis-decoding it.

- mesh.rs: Xbg7Model::from_xpr2 -> GameMesh { positions, normals, uvs,
  indices }; standalone f16->f32; unit + real-disc tests (weapon decodes
  to 215v/364t with unit normals + in-range UVs; DeltaSaber body
  declined).
- texture.rs: from_xpr2_index / texture_names so the viewer can pick a
  model's _col albedo map.
- viewer: loose .xpr with decodable XBG7 spawns Bevy meshes (real normals,
  double-sided) textured with the albedo, framed by the orbit camera; the
  central egui panel goes transparent so the 3D scene shows through.

Complex multi-stream body meshes (DeltaSaber f004, other vertex layouts)
remain undecoded and are cleanly declined — next target is dynamic RE.

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

crates/sylpheed-formats/src/texture.rs

@@ -221,6 +221,45 @@ impl X360Texture {
     /// 3. Read its GPUTEXTURE_FETCH_CONSTANT (GPUFC) at descriptor +0x18
     /// 4. De-tile the pixel data (if tiled) → linear layout
     pub fn from_xpr2(bytes: &[u8]) -> Result<Self, TextureError> {
+        // XPR files are frequently PACKS of many textures; XPR_RES_INDEX picks
+        // the Nth texture resource (default 0) for RE/browse validation.
+        let want = std::env::var("XPR_RES_INDEX")
+            .ok()
+            .and_then(|v| v.trim().parse::<usize>().ok())
+            .unwrap_or(0);
+        Self::from_xpr2_index(bytes, want)
+    }
+
+    /// List the names of the texture resources (`TX2D` / `TXCM`) in an XPR2
+    /// container, in directory order — the same order [`from_xpr2_index`]
+    /// selects by. Non-texture resources (e.g. `XBG7`) are skipped.
+    pub fn texture_names(bytes: &[u8]) -> Vec<String> {
+        use std::io::Cursor;
+        let mut cur = Cursor::new(bytes);
+        let Ok(header) = Xpr2Header::read(&mut cur) else {
+            return Vec::new();
+        };
+        let mut names = Vec::new();
+        for _ in 0..header.num_resources {
+            let Ok(e) = Xpr2ResourceEntry::read(&mut cur) else {
+                break;
+            };
+            if e.is_texture() || e.is_cubemap() {
+                const DIR_BASE: usize = 0x10;
+                let no = e.name_offset as usize + DIR_BASE;
+                let name = bytes
+                    .get(no..)
+                    .and_then(|s| s.iter().position(|&c| c == 0).map(|p| &s[..p]))
+                    .map(|s| String::from_utf8_lossy(s).into_owned())
+                    .unwrap_or_default();
+                names.push(name);
+            }
+        }
+        names
+    }
+
+    /// Decode the `want`-th texture resource (`TX2D` / `TXCM`, directory order).
+    pub fn from_xpr2_index(bytes: &[u8], want: usize) -> Result<Self, TextureError> {
         use std::io::Cursor;
         let mut cur = Cursor::new(bytes);
 
@@ -236,12 +275,6 @@ impl X360Texture {
         // Select a texture resource. TX2D = 2D texture; TXCM = cubemap
         // (skybox / backdrop) — same 52-byte descriptor + GPUFC layout, but the
         // pixel section holds 6 faces. For a preview we decode face 0.
-        // XPR files are frequently PACKS of many textures; XPR_RES_INDEX picks
-        // the Nth texture resource (default 0) for RE/browse validation.
-        let want = std::env::var("XPR_RES_INDEX")
-            .ok()
-            .and_then(|v| v.trim().parse::<usize>().ok())
-            .unwrap_or(0);
         let tex_entry = entries.iter()
             .filter(|e| e.is_texture() || e.is_cubemap())
             .nth(want)