Slightly improve performance and maintainability in SequentialGuidValueGenerator

[rameel]

Replace unnecessary byte copying with direct in-place modification.

Guid uses a sequential layout where the first 8 bytes require endianness handling on big-endian systems when reading from or writing to a byte array (e.g., via MemoryMarshal or Guid constructors). The remaining 8 bytes are interpreted as-is, regardless of endianness.

Since we only modify the last 8 bytes of the Guid, byte order does not affect the result, so we can:

• Eliminate intermediate copies
• Skip unnecessary byte order conversions
• Safely use MemoryMarshal.AsBytes for direct access

The optimization maintains identical behavior while being more efficient.