viewer: preview reassembled UI screens in the PAK browser

When an opened GP_*.pak RATC entry is a .rat build, compose the screen
(ui_layout::compose_build) and show it atop the RATC detail, labelled
'UI screen', with the sprite children below. Reuses the existing pak-open +
T8aD-decode path — no new browser/threading. Entry summaries mark UI builds.

cargo check -p sylpheed-viewer: clean.

crates/sylpheed-viewer/src/iso_loader.rs

@@ -160,8 +160,9 @@ pub enum PakContent {
     T8ad(ImageRgba),
     /// An LSTA sprite list — inline T8aD frames.
     Lsta(Vec<ImageRgba>),
-    /// A RATC bundle — its listed children (T8aD children carry a decoded image).
-    Ratc(Vec<RatcEntry>),
+    /// A RATC bundle — its listed children (T8aD children carry a decoded
+    /// image), plus the reassembled UI screen when the bundle is a `.rat` build.
+    Ratc(Vec<RatcEntry>, Option<ImageRgba>),
     /// Plain-text / XML payload, with its encoding label.
     Text { text: String, encoding: String },
 }
@@ -192,6 +193,13 @@ impl ImageRgba {
             rgba: img.rgba,
         }
     }
+    fn from_composed(s: sylpheed_formats::ui_layout::ComposedScreen) -> Self {
+        Self {
+            width: s.width,
+            height: s.height,
+            rgba: s.rgba,
+        }
+    }
     fn pixels(&self) -> usize {
         (self.width as usize) * (self.height as usize)
     }
@@ -1469,7 +1477,10 @@ fn classify_content(payload: &[u8]) -> PakContent {
                 })
                 .collect();
             if !entries.is_empty() {
-                return PakContent::Ratc(entries);
+                // If this bundle is a `.rat` UI build, reassemble the screen.
+                let ui_screen = sylpheed_formats::ui_layout::compose_build(payload, false)
+                    .map(ImageRgba::from_composed);
+                return PakContent::Ratc(entries, ui_screen);
             }
         }
     }

crates/sylpheed-viewer/src/ui.rs

@@ -672,7 +672,9 @@ fn draw_pak_browser(ui: &mut egui::Ui, pak: &mut PakView) {
                     PakContent::Png(img) => draw_png_detail(ui, row, img, img_tex),
                     PakContent::T8ad(img) => draw_t8ad_detail(ui, row, img, img_tex),
                     PakContent::Lsta(frames) => draw_lsta_detail(ui, row, frames, img_tex),
-                    PakContent::Ratc(children) => draw_ratc_detail(ui, row, children, img_tex),
+                    PakContent::Ratc(children, ui_screen) => {
+                        draw_ratc_detail(ui, row, children, ui_screen.as_ref(), img_tex)
+                    }
                     PakContent::Text { text, encoding } => {
                         draw_text_detail(ui, row, text, encoding)
                     }
@@ -699,7 +701,10 @@ fn row_kind(row: &crate::iso_loader::PakRow) -> String {
         PakContent::Png(img) => format!("PNG {}×{}", img.width, img.height),
         PakContent::T8ad(img) => format!("T8aD {}×{}", img.width, img.height),
         PakContent::Lsta(f) => format!("LSTA · {} sprite(s)", f.len()),
-        PakContent::Ratc(c) => format!("RATC · {} item(s)", c.len()),
+        PakContent::Ratc(c, screen) => {
+            let s = if screen.is_some() { " · UI screen" } else { "" };
+            format!("RATC · {} item(s){s}", c.len())
+        }
         PakContent::Text { .. } => "text".into(),
         PakContent::None => row.identity.clone(),
     }
@@ -939,21 +944,47 @@ fn draw_ratc_detail(
     ui: &mut egui::Ui,
     row: &crate::iso_loader::PakRow,
     children: &[crate::iso_loader::RatcEntry],
+    ui_screen: Option<&ImageRgba>,
     img_tex: &mut ImgCache,
 ) {
     ui.horizontal(|ui| {
         ui.heading("RATC bundle");
         ui.separator();
         ui.label(format!("{} item(s)", children.len()));
+        if ui_screen.is_some() {
+            ui.separator();
+            ui.strong("🖼 UI screen");
+        }
         ui.separator();
         ui.weak("colours unverified");
     });
     ui.separator();
 
-    let refs: Vec<&ImageRgba> = children.iter().filter_map(|c| c.image.as_ref()).collect();
+    // Cache the reassembled screen (if any) as texture 0, then child thumbnails.
+    let mut refs: Vec<&ImageRgba> = Vec::new();
+    if let Some(s) = ui_screen {
+        refs.push(s);
+    }
+    let child_base = refs.len();
+    refs.extend(children.iter().filter_map(|c| c.image.as_ref()));
     ensure_textures(ui, row.hash, &refs, img_tex);
 
-    let mut img_i = 0;
+    // The reassembled screen, scaled to fit the panel width.
+    if ui_screen.is_some() {
+        if let Some(tex) = img_tex.as_ref().and_then(|(_, t)| t.first()) {
+            ui.label("Reassembled from this screen's .rat layout records:");
+            let sz = tex.size_vec2();
+            let scale = (ui.available_width() / sz.x.max(1.0)).min(1.0);
+            ui.add(egui::Image::new(egui::load::SizedTexture::new(
+                tex.id(),
+                [sz.x * scale, sz.y * scale],
+            )));
+            ui.weak("Frame/glow decorations (no .rat) and the live 3D background are omitted.");
+            ui.separator();
+        }
+    }
+
+    let mut img_i = child_base;
     for c in children {
         ui.horizontal(|ui| {
             if c.image.is_some() {