support for "owa", audio-only webm, was introduced in iOS 17.5
owa is a more compliant alternative to opus-caf from iOS 11,
which was technically limited to CBR opus, a limitation which
we ignored since it worked mostly fine for regular opus too
being the new officially-recommended way to do things,
we'll default to owa for iOS 18 and later, even though
iOS still has some bugs affecting our use specifically:
if a weba file is preloaded into a 2nd audio object,
safari will throw a spurious exception as playback is
initiated, even as the file is playing just fine
the `.ld` stuff is an attempt at catching and ignoring this
spurious error without eating any actual network exceptions
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