Add userland syscall resume path and tty input flush on exec

[pradosh-arduino]

Motivation

• Ensure exec does not inherit stale TTY input by flushing cooked input before launching a new userland program.
• Provide a safe kernel-resume/trampoline path for userland to return into kernel control flow from a syscall, preserving/restoring non-volatile registers and avoiding clobbering the kernel stack.
• Introduce a dedicated syscall stack and update TSS rsp0 to improve isolation of kernel-side syscall handling while userland is running.

Description

• Added a new tty_flush_input() prototype in source/includes/tty.h and implemented it in source/kernel/C/user-input/tty.c to clear the cooked ring buffer and reset the current line buffer length.
• Updated source/kernel/C/shell/commands/exec.c to include tty.h and call tty_flush_input() before invoking userland_exec() so the new process starts with a clean input state.
• Extended syscall entry assembly source/kernel/C/interrupts/syscalls-x86_64.asm to check the userland_should_return_kernel flag and, if set, load saved non-volatile registers and rsp/rip from userland_resume_* variables and jump to the saved kernel resume address instead of performing iretq back to userland.
• Modified source/kernel/C/userland.c to add the userland_should_return_kernel flag, userland_resume_* register save areas, a dedicated userland_syscall_stack, and logic to save current kernel registers and rsp into the resume slots before entering userland, switch kernel_stack_top and tss.rsp0 to the syscall stack while userland is running, and restore them on return; userland_prepare_exit() now signals the kernel resume via the flag rather than mutating a provided frame.

Testing

• Performed an automated build of the kernel image with make, which completed successfully.
• Ran the automated unit/regression test suite (make test / CI job that covers userland exec workflow), and the tests passed successfully and reported expected userland exec/exit behavior.
• Verified the automated kernel boot smoke test in CI passed with the new syscall stack and resume path enabled.

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Codex Task