feat: Implement keyboard input handling and shell functionality

- Added framebuffer initialization and retrieval functions in multiboot2.zig.
- Integrated framebuffer support into the boot sequence.
- Refactored keyboard driver to use a circular buffer for input and added blocking behavior for kata waiting on keyboard input.
- Enhanced kata management to support blocking and waking katas based on keyboard input.
- Introduced a new invocation for reading a character from the keyboard (getkeychar).
- Implemented a simple shell (ash) that reads user input and echoes it back.
- Updated syscall handling to include the new getkeychar invocation.
- Improved memory management for loading programs and handling user/kernel stacks.
- Added functionality to wake katas waiting for specific events, such as the dissolution of other katas.

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