The download transport preference defaulted to "auto", which resolves to Xet on
any machine whose hf_xet looks healthy, so in practice everyone downloaded over
Xet without ever choosing it. New installs now download over HTTPS: it resumes a
cancelled or dropped transfer, and it needs nothing but ordinary TLS.
Installs from before this default keep Auto, seeded once from positive evidence
of prior use (a row the user wrote, or a manifest from an earlier download) and
persisted, so updating changes nobody's downloads.
Settings > General gains a Downloads section with an HTTPS / Xet / Auto control,
the difference between the two spelled out, the reason Xet is unavailable when
hf_xet is missing, and what Auto resolves to on this machine.
use_xet defaulted to True in the download schemas, so a scripted caller that
stated no transport got Xet whatever the preference said. It is optional now,
and a request stating neither field follows the install's setting.
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