Unlinking before re-creating fixes the premature-EOF spin on unix, but Windows
refuses to remove a file another handle still has open and returns a sharing
violation. Because Create surfaces that error, every cache miss on a path with a
live reader would have failed outright - worse than the spin it was meant to fix.
Split the create behind a build tag: unix unlinks for a fresh inode, Windows keeps
truncating in place and stays exposed to the spin, which is the behaviour it
already had. The unix-only spec is skipped there.
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