fix: use %w for error wrapping in WritePacket

[Yanhu007]

Fixes #1213

Problem

WritePacket in pcapgo/write.go uses %v to format the error from writePacketHeader:

return fmt.Errorf("error writing packet header: %v", err)

This converts the error to a string and loses the original error type, preventing errors.Is and errors.As from detecting underlying system errors (e.g. syscall.ENOSPC for disk full).

Fix

Change %v to %w to properly wrap the error:

return fmt.Errorf("error writing packet header: %w", err)

This preserves the error chain, allowing callers to use errors.Is(err, syscall.ENOSPC) and similar checks.

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