chore(billyfs): silence stdmethods warning on Seek
The Seek method on billyFile must return expsys.Errno to satisfy
wazero's experimental/sys.File interface, but Errno implements error,
which trips go vet's stdmethods near-miss check for io.Seeker. Add
//nolint:stdmethods (honored by golangci-lint) and a short comment so
future readers don't try to "fix" the signature and break the
interface.
Assisted-by: Claude Opus 4.7 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>