MemoryBlock now supports Double reads and writes

Add ReadDouble(offset As Integer) As Double and WriteDouble(offset As Integer, value As Double) to MemoryBlock so Objo code can read and write 64-bit IEEE 754 binary64 doubles to and from raw byte buffers, following the block's existing LittleEndian byte-order setting and offset/bounds validation rules.

This makes it possible to work with binary file formats, network payloads, and other packed data that contains doubles without hand-rolling byte shuffling.