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-xex/Cargo.toml

@@ -8,6 +8,7 @@ license.workspace = true
 xenia-types = { workspace = true }
 xenia-memory = { workspace = true }
 tracing = { workspace = true }
+metrics = { workspace = true }
 byteorder = { workspace = true }
 thiserror = { workspace = true }
 anyhow = { workspace = true }