feat(boot+shell): add bootable disk image, unified scheduler and interactive user-space shell

- Implement Limine v8 boot protocol requests in dedicated PT_LOAD RW segment
- Configure IOPL=3 and SSE in arch setup for Ring 3 drivers and libc
- Unify scheduler context switching via switch_to and preempt_tick
- Support dual-source keyboard/serial input in sys_key_read
- Add automated 'make image' and 'make run' targets for instant QEMU launch
- Complete interactive UNIX-like shell (osh) with built-in commands & ramdisk

servers/sh/main.cpp

@@ -202,6 +202,11 @@ static void handle_command(char *cmd) {
 }
 
 extern "C" void _start() {
+    // Let root servers finish early boot logging before taking over the terminal
+    for (int i = 0; i < 6; i++) {
+        sys_yield();
+    }
+
     sys_log_debug("\033[2J\033[H", 7);
     puts("===============================================================================");
     puts("         opencoreC High-Performance Microkernel Operating System (x86_64)      ");