xenia-vfs/xex: cache full disc tree; instrument XEX load

DiscImageDevice now walks the GDFX tree at open() and caches every
file/dir entry by full relative path; the previous root-only scan
returned ENOENT for any path under a subdirectory (dat/tables.pak,
media/x.wav). Lookups become O(n) over the cached vec.

xex::load_image gains a tracing span plus per-load metrics
(xex.load_image_ms histogram, xex.bytes_{in,out} counters) so the
observability subscriber the app installs can see decompression cost.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-vfs/src/lib.rs

@@ -16,7 +16,7 @@ pub enum VfsError {
 }
 
 /// A virtual filesystem entry (file or directory).
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct VfsEntry {
     pub name: String,
     pub is_directory: bool,