* perf(metadata): cache the per-build app metadata-file scan
* fix(metadata): invalidate cache on opengraph/twitter alt-text sidecars
scanMetadataFiles folds `*.alt.txt` sidecars into a static social image
route's altFilePath via an existsSync probe, but isMetadataRouteFile did
not treat them as route files. With the new per-appDir scan cache, adding
or removing an alt sidecar in dev no longer triggered invalidation, so
the cached scan kept a stale altFilePath. Match the sidecars in the dev
invalidation predicate so the metadata cache clears and re-scans.
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Co-authored-by: James Anderson <james@eli.cx>
Yunfei He