feat(viewer): directory-tree browser + plain-text viewer

Explorer drawer was a flat list of all 353 disc files and only textures had a
preview. Two additions:

1. Directory tree — the left drawer is now a collapsible `📁` tree, rebuilt each
   frame from the path list (ui.rs: build_tree / split_for_tree / render_dir).
   Filtering narrows leaves and auto-expands matches; `.p00…p04` data segments
   nest under a virtual `<stem>.pak` node. Selection still keys off the original
   file index, so the loader pipeline is untouched.

2. Plain-text viewer — config.ini and any file that sniffs as text now show
   read-only, selectable, monospace content with a Wrap toggle and an encoding
   label. Detection/decoding live in the Bevy-free formats crate:
   vfs::is_probably_text + vfs::decode_text (BOM-aware UTF-8/UTF-16LE/BE, BOM-less
   UTF-16LE heuristic, NUL-reject + 95%-printable fallback) — pure std, WASM-safe,
   unit-tested (6 new tests), and reused by the CLI sniffer (new cyan `txt`).

New TextPreview resource (registered unconditionally so it exists on wasm);
populated in apply_loaded_texture with a 2 MiB char-boundary-safe cap. Also fixes
a latent bug: switching from a texture to a non-texture left stale preview state —
both texture and text state now reset on every selection.

ViewerState gains text_wrap. Formats tests: 22 pass; workspace compiles.

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

crates/sylpheed-viewer/src/iso_loader.rs

@@ -90,6 +90,22 @@ pub struct FileInfo {
     pub detected_format: Option<FileFormat>,
 }
 
+/// Decoded contents of the currently-selected text file, shown in the central
+/// panel. `content` is `None` whenever the selection isn't text.
+#[derive(Resource, Default)]
+pub struct TextPreview {
+    /// Decoded UTF-8 (any BOM / UTF-16 already resolved), or `None`.
+    pub content: Option<String>,
+    /// Encoding label for the header, e.g. "UTF-16LE (BOM)".
+    pub encoding: String,
+    /// True when the file was larger than `MAX_TEXT_BYTES` and got clipped.
+    pub truncated: bool,
+}
+
+/// Cap on the decoded text we hand to egui — its text layout cost grows
+/// super-linearly, and disc config files are only a few KiB anyway.
+const MAX_TEXT_BYTES: usize = 2 * 1024 * 1024;
+
 // ── SystemSet label ───────────────────────────────────────────────────────────
 
 /// `draw_viewer_ui` runs `.after(IsoLoaderSystemSet)` to see the frame's
@@ -157,7 +173,10 @@ impl Plugin for IsoLoaderPlugin {
             .add_event::<FileSelected>()
             .init_resource::<IsoState>()
             .init_resource::<TexturePreview>()
-            .init_resource::<FileInfo>();
+            .init_resource::<FileInfo>()
+            // Registered unconditionally (even on wasm, where the load systems
+            // below are cfg'd out) so the UI can always read it.
+            .init_resource::<TextPreview>();
 
         #[cfg(not(target_arch = "wasm32"))]
         {
@@ -392,6 +411,7 @@ fn poll_loader_channel(
 fn apply_loaded_texture(
     mut pending: ResMut<PendingFileBytes>,
     mut preview: ResMut<TexturePreview>,
+    mut text_preview: ResMut<TextPreview>,
     mut file_info: ResMut<FileInfo>,
     mut images: ResMut<Assets<Image>>,
     mut contexts: bevy_egui::EguiContexts,
@@ -412,6 +432,9 @@ fn apply_loaded_texture(
         images.remove(handle.id());
     }
     *preview = TexturePreview::default();
+    // Clear stale text too, so switching files never shows a previous file's
+    // content in the wrong panel.
+    *text_preview = TextPreview::default();
 
     // Populate FileInfo for the status / info panel.
     file_info.name = path
@@ -422,6 +445,25 @@ fn apply_loaded_texture(
     file_info.size_bytes = bytes.len();
     file_info.detected_format = Some(fmt);
 
+    if fmt == FileFormat::Text {
+        let (mut text, encoding) = sylpheed_formats::vfs::decode_text(&bytes);
+        let truncated = text.len() > MAX_TEXT_BYTES;
+        if truncated {
+            // Cut on a char boundary so we never split a codepoint.
+            let end = text
+                .char_indices()
+                .take_while(|(i, _)| *i < MAX_TEXT_BYTES)
+                .last()
+                .map(|(i, c)| i + c.len_utf8())
+                .unwrap_or(0);
+            text.truncate(end);
+        }
+        text_preview.content = Some(text);
+        text_preview.encoding = encoding.to_string();
+        text_preview.truncated = truncated;
+        return; // FileInfo + TextPreview are sufficient for text files.
+    }
+
     if fmt != FileFormat::Xpr2Texture {
         return; // Not a texture — FileInfo is sufficient
     }