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>
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