🧩 Add support for user mode (ring 3) execution (64 bit)

[IRodriguez13]

• Currently, all processes run in ring 0. There is no separation between kernel and user space, which prevents proper privilege isolation and memory protection. Placeholder functions like switch_to_user_mode_x86() exist but are not implemented.

Tasks to consider:

• Set up GDT with user segments

• Implement transition to ring 3 via iret or syscall/sysret

• Define user-space entry point and stack

• Ensure memory protection between processes

• This is essential for sandboxing, syscall testing, and future security features.

[IRodriguez13]

🛠 Contribution Reminder: To contribute to this issue, please follow these steps:

• Fork the upstream repository

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• Create a branch named feat/issue-name based on your fork.

• Make your changes and commit them to that branch.

• Open a pull request targeting the dev branch — never mainline.

This helps keep contributions organized and ensures stability in the mainline branch. Thanks for helping improve the kernel!