https://gitlab.synchro.net/main/sbbs/-/commit/2b030be7a3717cf215f68745
Added Files:
exec/load/xtrn_mirror.js xtrn/tests/xtrn_mirror_test.js
Log Message:
xtrn: add exec/load/xtrn_mirror.js, a mirror-fallback download helper

Door asset fetchers download game data from a dozen upstream hosts nobody
here controls. When one goes away, renames a release, or refuses the
request, the door cannot be installed at all and the sysop's only
recourse is to find the file by hand. This adds a shared helper that
retries against Synchronet's asset mirror when the publisher fails.

The fallback fires on an unreachable host, a non-2xx response, and --
via the caller's optional verify callback -- a checksum mismatch. That
last case is the point: without a check, a second source would be
trusted exactly as blindly as the first, so adding one would weaken
every fetcher that used it rather than strengthen it.

HTTPRequest.Download() streams a 404 error body to disk before returning
0, so a failed attempt removes the file it wrote; otherwise an HTML
error page would reach Archive() claiming to be a game archive.

No fetcher calls it yet.

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