Fixes#6962
I believe this is an upstream bug
(https://github.com/ziglang/zig/issues/23454), where Zig is allowing
extern unions to be tagged when created via type reification. This
results in a CValue that has an extra trailing byte (the tag).
This wasn't causing any noticeable issues for Ghostty for some reason
but others using our pattern were seeing issues. And I did confirm that
our CValue was indeed tagged and was the wrong byte size. I assume Swift
was just ignoring it because it was extra data. I don't know, but we
should fix this in general for libghostty.
e19b5a150a