Add userland resume path and tty input flush on exec

[pradosh-arduino]

Motivation

• Ensure clean terminal state when launching a new userland program and provide a safe mechanism for userland to return control to the kernel outside the normal iretq path.
• Preserve callee-saved registers and kernel stack context so the kernel can resume execution atomically when a userland process requests to return.

Description

• Added tty_flush_input() to the TTY API (tty.h) and implemented it to clear the cooked ring buffer and reset the current line (tty.c).
• Call tty_flush_input() from the shell exec command (exec.c) before invoking userland_exec to clear leftover input.
• Introduced volatile resume state variables and a userland_should_return_kernel flag in userland.c, and save callee-saved registers (rbx, rbp, r12..r15) plus kernel rsp/rip before entering userland.
• Updated userland_prepare_exit() to signal return by setting userland_should_return_kernel instead of directly patching a syscall frame.
• Modified syscall entry assembly (syscalls-x86_64.asm) to check userland_should_return_kernel and, if set, restore saved registers and rsp/rip and jump directly to the saved kernel resume address; otherwise perform the normal iretq return.
• Clear resume state after returning to kernel and ensure userland_running/exit code handling remains intact.

Testing

• Built the kernel using make which completed successfully without compilation errors.
• Booted the kernel under QEMU with a minimal automated smoke test that runs the shell and executes a small userland binary via exec, and observed the kernel print the process exit code and return to the shell, indicating the resume/exit path and TTY flush behaved as expected.

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Codex Task