Handle non-sequential writes in MultiuserFile

[djw8605]

Disable checksum calculation for non-sequential writes and update the offset correctly after writing.

[Copilot]

Pull request overview

This PR updates MultiuserFile::Write to tolerate non-sequential writes by disabling on-write checksum calculation instead of rejecting the write, and it corrects how the next expected write offset is tracked.

Changes:

• Disable checksum calculation when a non-sequential write is detected (instead of returning -ENOTSUP).
• Update m_nextoff to offset + bytes_written after each successful write.

Comments suppressed due to low confidence (1)

src/multiuser.cpp:191

• Checksum updates use the requested size instead of the actual number of bytes written (result). If the underlying write is partial, the checksum state (and its internal offset) will advance too far and the final checksum will not match the file contents. Update the checksum with exactly the bytes successfully written (and only when result > 0).

    auto result = m_wrapped->Write(buffer, offset, size);
    if (result >= 0) {m_nextoff = offset + result;}
    if (m_state)
    {
        m_state->Update(static_cast<const unsigned char*>(buffer), size);
    }

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